From 765db115d6373163c9db952b08b610362064ef0b Mon Sep 17 00:00:00 2001
From: l-lejman <l.lejman@cksource.com>
Date: Fri, 9 May 2025 12:21:59 +0200
Subject: [PATCH 1/6] Fix manual tests language.ui value.

---
 packages/ckeditor5-core/src/context.ts        | 36 ++++++++++++++---
 packages/ckeditor5-core/tests/context.js      | 28 +++++++++++--
 .../ckeditor5-core/tests/editor/editor.js     | 40 ++++++++++++++++---
 3 files changed, 90 insertions(+), 14 deletions(-)

diff --git a/packages/ckeditor5-core/src/context.ts b/packages/ckeditor5-core/src/context.ts
index a62feeab1fa..80f324bb5de 100644
--- a/packages/ckeditor5-core/src/context.ts
+++ b/packages/ckeditor5-core/src/context.ts
@@ -12,6 +12,7 @@ import {
 	Collection,
 	CKEditorError,
 	Locale,
+	global,
 	type LocaleTranslate
 } from '@ckeditor/ckeditor5-utils';
 
@@ -160,11 +161,36 @@ export default class Context {
 
 		const languageConfig = this.config.get( 'language' ) || {};
 
-		this.locale = new Locale( {
-			uiLanguage: typeof languageConfig === 'string' ? languageConfig : languageConfig.ui,
-			contentLanguage: this.config.get( 'language.content' ),
-			translations
-		} );
+		if ( !translations && global.window.CKEDITOR_TRANSLATIONS ) {
+			/**
+			 * Function _translate from translation-service.ts gets translations from
+			 * global.window.CKEDITOR_TRANSLATIONS when translations injected into Locale are empty.
+			 * Value of global.window.CKEDITOR_TRANSLATIONS is provided by CKEditorTranslationsPlugin from
+			 * ckeditor5-dev-translations package (for example in manual tests) when --language flag is used.
+			 * Function _translate is called often and has no access to the editing editor config, so here is a better place to
+			 * check if translations will be taken from dev-translations, and if yes – update config.language.ui value.
+			 */
+			const devTranslations = global.window.CKEDITOR_TRANSLATIONS;
+			const uiLanguageFromConfig = typeof languageConfig === 'string' ? languageConfig : languageConfig.ui;
+			const hasMatchingTranslations = devTranslations[ uiLanguageFromConfig! ];
+			const defaultDevTranslationsLanguage = Object.keys( devTranslations )[ 0 ];
+
+			this.locale = new Locale( {
+				uiLanguage: hasMatchingTranslations ? uiLanguageFromConfig : defaultDevTranslationsLanguage,
+				contentLanguage: this.config.get( 'language.content' ),
+				translations: global.window.CKEDITOR_TRANSLATIONS
+			} );
+
+			if ( !hasMatchingTranslations && this.config.get( 'language.ui' ) !== defaultDevTranslationsLanguage ) {
+				this.config.define( 'language.ui', defaultDevTranslationsLanguage );
+			}
+		} else {
+			this.locale = new Locale( {
+				uiLanguage: typeof languageConfig === 'string' ? languageConfig : languageConfig.ui,
+				contentLanguage: this.config.get( 'language.content' ),
+				translations
+			} );
+		}
 
 		this.t = this.locale.t;
 
diff --git a/packages/ckeditor5-core/tests/context.js b/packages/ckeditor5-core/tests/context.js
index 83935c91864..f618cf5d948 100644
--- a/packages/ckeditor5-core/tests/context.js
+++ b/packages/ckeditor5-core/tests/context.js
@@ -12,7 +12,7 @@ import Locale from '@ckeditor/ckeditor5-utils/src/locale.js';
 import VirtualTestEditor from './_utils/virtualtesteditor.js';
 import CKEditorError from '@ckeditor/ckeditor5-utils/src/ckeditorerror.js';
 
-/* globals document */
+/* globals document, window */
 
 describe( 'Context', () => {
 	describe( 'config', () => {
@@ -93,26 +93,46 @@ describe( 'Context', () => {
 		} );
 
 		it( 'is configured with the config.language (UI and the content)', () => {
-			const context = new Context( { language: 'pl' } );
+			const context = new Context( { language: 'pl', translations: { pl: {} } } );
 
 			expect( context.locale.uiLanguage ).to.equal( 'pl' );
 			expect( context.locale.contentLanguage ).to.equal( 'pl' );
 		} );
 
 		it( 'is configured with the config.language (different for UI and the content)', () => {
-			const context = new Context( { language: { ui: 'pl', content: 'ar' } } );
+			const context = new Context( { language: { ui: 'pl', content: 'ar' }, translations: { pl: {} } } );
 
 			expect( context.locale.uiLanguage ).to.equal( 'pl' );
 			expect( context.locale.contentLanguage ).to.equal( 'ar' );
 		} );
 
 		it( 'is configured with the config.language (just the content)', () => {
-			const context = new Context( { language: { content: 'ar' } } );
+			const context = new Context( { language: { content: 'ar' }, translations: { pl: {} } } );
 
 			expect( context.locale.uiLanguage ).to.equal( 'en' );
 			expect( context.locale.contentLanguage ).to.equal( 'ar' );
 		} );
 
+		it( 'is configured with the default config.language (when no translations provided)', () => {
+			const context = new Context( { language: 'pl' } );
+
+			expect( context.locale.uiLanguage ).to.equal( 'en' );
+			expect( context.locale.contentLanguage ).to.equal( 'en' );
+		} );
+
+		it( 'is configured with config.language (when no translations provided and dev-translations are matching)', () => {
+			window.CKEDITOR_TRANSLATIONS = {
+				en: { dictionary: {
+					key: ''
+				},
+				getPluralForm: () => '' }
+			};
+			const context = new Context( { language: { ui: 'en', content: 'en' } } );
+
+			expect( context.locale.uiLanguage ).to.equal( 'en' );
+			expect( context.locale.contentLanguage ).to.equal( 'en' );
+		} );
+
 		it( 'is configured with the config.translations', () => {
 			const context = new Context( {
 				translations: {
diff --git a/packages/ckeditor5-core/tests/editor/editor.js b/packages/ckeditor5-core/tests/editor/editor.js
index 10674e4d58a..ebc23a7e9e7 100644
--- a/packages/ckeditor5-core/tests/editor/editor.js
+++ b/packages/ckeditor5-core/tests/editor/editor.js
@@ -353,7 +353,13 @@ describe( 'Editor', () => {
 
 		it( 'should use locale instance with a proper configuration passed as the argument to the constructor', () => {
 			const editor = new TestEditor( {
-				language: 'pl'
+				language: 'pl',
+				translations: [ {
+					pl: {
+						dictionary: [],
+						getPluralForm: sinon.spy()
+					}
+				} ]
 			} );
 
 			expect( editor.locale ).to.have.property( 'uiLanguage', 'pl' );
@@ -362,7 +368,13 @@ describe( 'Editor', () => {
 
 		it( 'should use locale instance with a proper configuration set as the defaultConfig option on the constructor', () => {
 			TestEditor.defaultConfig = {
-				language: 'pl'
+				language: 'pl',
+				translations: [ {
+					pl: {
+						dictionary: [],
+						getPluralForm: sinon.spy()
+					}
+				} ]
 			};
 
 			const editor = new TestEditor();
@@ -373,7 +385,13 @@ describe( 'Editor', () => {
 
 		it( 'should prefer the language passed as the argument to the constructor instead of the defaultConfig if both are set', () => {
 			TestEditor.defaultConfig = {
-				language: 'de'
+				language: 'de',
+				translations: [ {
+					pl: {
+						dictionary: [],
+						getPluralForm: sinon.spy()
+					}
+				} ]
 			};
 
 			const editor = new TestEditor( {
@@ -386,10 +404,22 @@ describe( 'Editor', () => {
 
 		it( 'should prefer the language from the context instead of the constructor config or defaultConfig if all are set', async () => {
 			TestEditor.defaultConfig = {
-				language: 'de'
+				language: 'de',
+				translations: [ {
+					pl: {
+						dictionary: [],
+						getPluralForm: sinon.spy()
+					}
+				} ]
 			};
 
-			const context = await Context.create( { language: 'pl' } );
+			const context = await Context.create( {
+				language: 'pl',
+				translations: [ { pl: {
+					dictionary: [],
+					getPluralForm: sinon.spy()
+				} } ]
+			} );
 			const editor = new TestEditor( { context, language: 'ru' } );
 
 			expect( editor.locale ).to.have.property( 'uiLanguage', 'pl' );

From 5bba8dc54e98c1405db39b157b8671f76094b241 Mon Sep 17 00:00:00 2001
From: l-lejman <l.lejman@cksource.com>
Date: Fri, 9 May 2025 15:53:34 +0200
Subject: [PATCH 2/6] Adjust tests to manuals language change.

---
 .../ckeditor5-alignment/tests/alignmentui.js     |  8 ++++++++
 packages/ckeditor5-ckbox/tests/ckboxutils.js     |  4 ++++
 .../ckeditor5-ckfinder/tests/ckfindercommand.js  |  6 +++++-
 packages/ckeditor5-core/src/context.ts           |  5 +++--
 packages/ckeditor5-font/tests/ui/colorui.js      |  7 ++++++-
 packages/ckeditor5-indent/tests/indentui.js      |  6 +++++-
 .../legacytodolist/legacytodolistediting.js      |  4 ++++
 .../tablecellpropertiesediting.js                |  6 +++++-
 .../tablecolumnresizeediting.js                  |  6 +++++-
 packages/ckeditor5-table/tests/tablekeyboard.js  |  6 +++++-
 packages/ckeditor5-ui/tests/badge/badge.js       |  6 +++++-
 .../tests/editorui/evaluationbadge.js            |  6 +++++-
 .../ckeditor5-ui/tests/editorui/poweredby.js     |  6 +++++-
 packages/ckeditor5-undo/tests/undoui.js          | 16 +++++++++++++---
 14 files changed, 78 insertions(+), 14 deletions(-)

diff --git a/packages/ckeditor5-alignment/tests/alignmentui.js b/packages/ckeditor5-alignment/tests/alignmentui.js
index 3c9b758d0d2..793431152a3 100644
--- a/packages/ckeditor5-alignment/tests/alignmentui.js
+++ b/packages/ckeditor5-alignment/tests/alignmentui.js
@@ -305,6 +305,10 @@ describe( 'Alignment UI', () => {
 						content: 'ar',
 						ui: 'ar'
 					},
+					translations: [ { ar: {
+						dictionary: [],
+						getPluralForm: sinon.spy()
+					} } ],
 					plugins: [ AlignmentEditing, AlignmentUI ]
 				} );
 
@@ -439,6 +443,10 @@ describe( 'Alignment UI', () => {
 						language: {
 							content: 'ar'
 						},
+						translations: [ { ar: {
+							dictionary: [],
+							getPluralForm: sinon.spy()
+						} } ],
 						plugins: [ AlignmentEditing, AlignmentUI ],
 						alignment: { options: [ 'center', 'justify' ] }
 					} )
diff --git a/packages/ckeditor5-ckbox/tests/ckboxutils.js b/packages/ckeditor5-ckbox/tests/ckboxutils.js
index 9aa0be03b52..b21446af40e 100644
--- a/packages/ckeditor5-ckbox/tests/ckboxutils.js
+++ b/packages/ckeditor5-ckbox/tests/ckboxutils.js
@@ -314,6 +314,10 @@ describe( 'CKBoxUtils', () => {
 		it( 'should set default values', async () => {
 			const editor = await createTestEditor( {
 				language: 'pl',
+				translations: [ { pl: {
+					dictionary: [],
+					getPluralForm: sinon.spy()
+				} } ],
 				cloudServices: {
 					tokenUrl: 'http://cs.example.com'
 				}
diff --git a/packages/ckeditor5-ckfinder/tests/ckfindercommand.js b/packages/ckeditor5-ckfinder/tests/ckfindercommand.js
index b3ad9362047..9466d163959 100644
--- a/packages/ckeditor5-ckfinder/tests/ckfindercommand.js
+++ b/packages/ckeditor5-ckfinder/tests/ckfindercommand.js
@@ -262,7 +262,11 @@ describe( 'CKFinderCommand', () => {
 			return VirtualTestEditor
 				.create( {
 					plugins: [ Paragraph, ImageBlockEditing, ImageUploadEditing, LinkEditing, Notification, ClipboardPipeline ],
-					language: 'pl'
+					language: 'pl',
+					translations: [ { pl: {
+						dictionary: [],
+						getPluralForm: sinon.spy()
+					} } ]
 				} )
 				.then( newEditor => {
 					editor = newEditor;
diff --git a/packages/ckeditor5-core/src/context.ts b/packages/ckeditor5-core/src/context.ts
index 80f324bb5de..e5c046bafa3 100644
--- a/packages/ckeditor5-core/src/context.ts
+++ b/packages/ckeditor5-core/src/context.ts
@@ -20,6 +20,7 @@ import PluginCollection from './plugincollection.js';
 import type Editor from './editor/editor.js';
 import type { LoadedPlugins, PluginConstructor } from './plugin.js';
 import type { EditorConfig } from './editor/editorconfig.js';
+import { cloneDeep } from 'es-toolkit/compat';
 
 /**
  * Provides a common, higher-level environment for solutions that use multiple {@link module:core/editor/editor~Editor editors}
@@ -170,7 +171,7 @@ export default class Context {
 			 * Function _translate is called often and has no access to the editing editor config, so here is a better place to
 			 * check if translations will be taken from dev-translations, and if yes – update config.language.ui value.
 			 */
-			const devTranslations = global.window.CKEDITOR_TRANSLATIONS;
+			const devTranslations = cloneDeep( global.window.CKEDITOR_TRANSLATIONS );
 			const uiLanguageFromConfig = typeof languageConfig === 'string' ? languageConfig : languageConfig.ui;
 			const hasMatchingTranslations = devTranslations[ uiLanguageFromConfig! ];
 			const defaultDevTranslationsLanguage = Object.keys( devTranslations )[ 0 ];
@@ -178,7 +179,7 @@ export default class Context {
 			this.locale = new Locale( {
 				uiLanguage: hasMatchingTranslations ? uiLanguageFromConfig : defaultDevTranslationsLanguage,
 				contentLanguage: this.config.get( 'language.content' ),
-				translations: global.window.CKEDITOR_TRANSLATIONS
+				translations: devTranslations
 			} );
 
 			if ( !hasMatchingTranslations && this.config.get( 'language.ui' ) !== defaultDevTranslationsLanguage ) {
diff --git a/packages/ckeditor5-font/tests/ui/colorui.js b/packages/ckeditor5-font/tests/ui/colorui.js
index 36c7441eab2..f4fd5b1b213 100644
--- a/packages/ckeditor5-font/tests/ui/colorui.js
+++ b/packages/ckeditor5-font/tests/ui/colorui.js
@@ -74,7 +74,12 @@ describe( 'ColorUI', () => {
 		return ClassicTestEditor
 			.create( element, {
 				plugins: [ Paragraph, TestColorPlugin, Undo ],
-				testColor: testColorConfig
+				testColor: testColorConfig,
+				language: 'en',
+				translations: [ { en: {
+					dictionary: [],
+					getPluralForm: sinon.spy()
+				} } ]
 			} )
 			.then( newEditor => {
 				editor = newEditor;
diff --git a/packages/ckeditor5-indent/tests/indentui.js b/packages/ckeditor5-indent/tests/indentui.js
index 2b4c67f6d72..5eb230c37ef 100644
--- a/packages/ckeditor5-indent/tests/indentui.js
+++ b/packages/ckeditor5-indent/tests/indentui.js
@@ -28,7 +28,11 @@ describe( 'IndentUI', () => {
 		rtlEditor = await ClassicTestEditor
 			.create( element, {
 				plugins: [ IndentUI, IndentEditing ],
-				language: 'ar'
+				language: 'ar',
+				translations: [ { ar: {
+					dictionary: [],
+					getPluralForm: sinon.spy()
+				} } ]
 			} );
 	} );
 
diff --git a/packages/ckeditor5-list/tests/legacytodolist/legacytodolistediting.js b/packages/ckeditor5-list/tests/legacytodolist/legacytodolistediting.js
index 5d7aba94497..3589efd8936 100644
--- a/packages/ckeditor5-list/tests/legacytodolist/legacytodolistediting.js
+++ b/packages/ckeditor5-list/tests/legacytodolist/legacytodolistediting.js
@@ -1166,6 +1166,10 @@ describe( 'LegacyTodoListEditing', () => {
 				return VirtualTestEditor
 					.create( {
 						language: 'ar',
+						translations: [ { ar: {
+							dictionary: [],
+							getPluralForm: sinon.spy()
+						} } ],
 						plugins: [ Paragraph, LegacyTodoListEditing, Typing, BoldEditing, BlockQuoteEditing ]
 					} )
 					.then( newEditor => {
diff --git a/packages/ckeditor5-table/tests/tablecellproperties/tablecellpropertiesediting.js b/packages/ckeditor5-table/tests/tablecellproperties/tablecellpropertiesediting.js
index 19511695417..260bbc6323e 100644
--- a/packages/ckeditor5-table/tests/tablecellproperties/tablecellpropertiesediting.js
+++ b/packages/ckeditor5-table/tests/tablecellproperties/tablecellpropertiesediting.js
@@ -756,7 +756,11 @@ describe( 'table cell properties', () => {
 					beforeEach( async () => {
 						editor = await VirtualTestEditor.create( {
 							plugins: [ TableCellPropertiesEditing, Paragraph, TableEditing ],
-							language: 'ar'
+							language: 'ar',
+							translations: [ { ar: {
+								dictionary: [],
+								getPluralForm: sinon.spy()
+							} } ]
 						} );
 
 						model = editor.model;
diff --git a/packages/ckeditor5-table/tests/tablecolumnresize/tablecolumnresizeediting.js b/packages/ckeditor5-table/tests/tablecolumnresize/tablecolumnresizeediting.js
index 7a9dfcd6a18..67692c23827 100644
--- a/packages/ckeditor5-table/tests/tablecolumnresize/tablecolumnresizeediting.js
+++ b/packages/ckeditor5-table/tests/tablecolumnresize/tablecolumnresizeediting.js
@@ -2064,7 +2064,11 @@ describe( 'TableColumnResizeEditing', () => {
 				}
 
 				editor = await createEditor( {
-					language: 'ar'
+					language: 'ar',
+					translations: [ { ar: {
+						dictionary: [],
+						getPluralForm: sinon.spy()
+					} } ]
 				} );
 
 				model = editor.model;
diff --git a/packages/ckeditor5-table/tests/tablekeyboard.js b/packages/ckeditor5-table/tests/tablekeyboard.js
index 57e33f81016..7a649f491da 100644
--- a/packages/ckeditor5-table/tests/tablekeyboard.js
+++ b/packages/ckeditor5-table/tests/tablekeyboard.js
@@ -3602,7 +3602,11 @@ describe( 'TableKeyboard', () => {
 				return VirtualTestEditor
 					.create( {
 						plugins: [ TableEditing, TableKeyboard, TableSelection, Paragraph, ImageBlockEditing, MediaEmbedEditing ],
-						language: 'ar'
+						language: 'ar',
+						translations: [ { ar: {
+							dictionary: [],
+							getPluralForm: sinon.spy()
+						} } ]
 					} )
 					.then( newEditor => {
 						editor = newEditor;
diff --git a/packages/ckeditor5-ui/tests/badge/badge.js b/packages/ckeditor5-ui/tests/badge/badge.js
index b84ec8afcd8..5bbbd185835 100644
--- a/packages/ckeditor5-ui/tests/badge/badge.js
+++ b/packages/ckeditor5-ui/tests/badge/badge.js
@@ -380,7 +380,11 @@ describe( 'Badge', () => {
 
 		it( 'should position to the left side if the UI language is RTL and no side was configured', async () => {
 			const editor = await createEditor( element, {
-				language: 'ar'
+				language: 'ar',
+				translations: [ { ar: {
+					dictionary: [],
+					getPluralForm: sinon.spy()
+				} } ]
 			} );
 
 			badge = new BadgeExtended( editor );
diff --git a/packages/ckeditor5-ui/tests/editorui/evaluationbadge.js b/packages/ckeditor5-ui/tests/editorui/evaluationbadge.js
index 421fdbbe50c..bcea49a1909 100644
--- a/packages/ckeditor5-ui/tests/editorui/evaluationbadge.js
+++ b/packages/ckeditor5-ui/tests/editorui/evaluationbadge.js
@@ -522,7 +522,11 @@ describe( 'EvaluationBadge', () => {
 		it( 'should position the badge to the left right if the UI language is RTL (and powered-by is on the left)', async () => {
 			const editor = await createEditor( element, {
 				language: 'ar',
-				licenseKey: developmentLicenseKey
+				licenseKey: developmentLicenseKey,
+				translations: [ { ar: {
+					dictionary: [],
+					getPluralForm: sinon.spy()
+				} } ]
 			} );
 
 			testUtils.sinon.stub( editor.ui.getEditableElement( 'main' ), 'getBoundingClientRect' ).returns( {
diff --git a/packages/ckeditor5-ui/tests/editorui/poweredby.js b/packages/ckeditor5-ui/tests/editorui/poweredby.js
index c9b855cc64e..58d76a81fc3 100644
--- a/packages/ckeditor5-ui/tests/editorui/poweredby.js
+++ b/packages/ckeditor5-ui/tests/editorui/poweredby.js
@@ -567,7 +567,11 @@ describe( 'PoweredBy', () => {
 
 		it( 'should position the to the left side if the UI language is RTL and no side was configured', async () => {
 			const editor = await createEditor( element, {
-				language: 'ar'
+				language: 'ar',
+				translations: [ { ar: {
+					dictionary: [],
+					getPluralForm: sinon.spy()
+				} } ]
 			} );
 
 			testUtils.sinon.stub( editor.ui.getEditableElement( 'main' ), 'getBoundingClientRect' ).returns( {
diff --git a/packages/ckeditor5-undo/tests/undoui.js b/packages/ckeditor5-undo/tests/undoui.js
index 7c040921938..6477ecbf105 100644
--- a/packages/ckeditor5-undo/tests/undoui.js
+++ b/packages/ckeditor5-undo/tests/undoui.js
@@ -23,7 +23,9 @@ describe( 'UndoUI', () => {
 		editorElement = document.createElement( 'div' );
 		document.body.appendChild( editorElement );
 
-		return ClassicTestEditor.create( editorElement, { plugins: [ UndoEditing, UndoUI ] } )
+		return ClassicTestEditor.create( editorElement, { plugins: [ UndoEditing, UndoUI ],
+			language: { ui: 'en' }
+		} )
 			.then( newEditor => {
 				editor = newEditor;
 			} );
@@ -110,7 +112,11 @@ describe( 'UndoUI', () => {
 				return ClassicTestEditor
 					.create( element, {
 						plugins: [ UndoEditing, UndoUI ],
-						language: 'ar'
+						language: 'ar',
+						translations: [ { ar: {
+							dictionary: [],
+							getPluralForm: sinon.spy()
+						} } ]
 					} )
 					.then( newEditor => {
 						const undoButton = newEditor.ui.componentFactory.create( 'undo' );
@@ -131,7 +137,11 @@ describe( 'UndoUI', () => {
 				return ClassicTestEditor
 					.create( element, {
 						plugins: [ UndoEditing, UndoUI ],
-						language: 'ar'
+						language: 'ar',
+						translations: [ { ar: {
+							dictionary: [],
+							getPluralForm: sinon.spy()
+						} } ]
 					} )
 					.then( newEditor => {
 						const redoButton = newEditor.ui.componentFactory.create( 'redo' );

From 868cc19ab78b4f571faa65cc4cc7a21e0a474547 Mon Sep 17 00:00:00 2001
From: l-lejman <l.lejman@cksource.com>
Date: Mon, 12 May 2025 15:07:30 +0200
Subject: [PATCH 3/6] Fix mathtype manual test and null handling in creating
 config objects.

---
 packages/ckeditor5-utils/src/config.ts | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/packages/ckeditor5-utils/src/config.ts b/packages/ckeditor5-utils/src/config.ts
index 85b2ceafe87..64864235d94 100644
--- a/packages/ckeditor5-utils/src/config.ts
+++ b/packages/ckeditor5-utils/src/config.ts
@@ -193,9 +193,9 @@ export default class Config<Cfg> {
 
 		// Iterate over parts to check if currently stored configuration has proper structure.
 		for ( const part of parts ) {
-			// If there is no object for specified part then create one.
-			if ( !isPlainObject( target[ part ] ) ) {
-				target[ part ] = Object.create( null );
+			// If target[part] is not an object or array, initialize as empty object.
+			if ( typeof target[ part ] !== 'object' || target[ part ] === null ) {
+				target[ part ] = {};
 			}
 
 			// Nested object becomes a target.
@@ -206,7 +206,7 @@ export default class Config<Cfg> {
 		if ( isPlainObject( value ) ) {
 			// We take care of proper config structure.
 			if ( !isPlainObject( target[ name ] ) ) {
-				target[ name ] = Object.create( null );
+				target[ name ] = {};
 			}
 
 			target = target[ name ];
@@ -218,7 +218,7 @@ export default class Config<Cfg> {
 		}
 
 		// Do nothing if we are defining configuration for non empty name.
-		if ( isDefine && typeof target[ name ] != 'undefined' ) {
+		if ( isDefine && typeof target[ name ] !== 'undefined' ) {
 			return;
 		}
 

From 521beafa60ac4965bd21d99949ead5e08b1e75e3 Mon Sep 17 00:00:00 2001
From: l-lejman <l.lejman@cksource.com>
Date: Wed, 25 Jun 2025 18:31:16 +0200
Subject: [PATCH 4/6] Remove prototype change from config and adjust comments.

---
 packages/ckeditor5-core/src/context.ts | 17 ++++++++++++-----
 packages/ckeditor5-utils/src/config.ts | 10 +++++-----
 2 files changed, 17 insertions(+), 10 deletions(-)

diff --git a/packages/ckeditor5-core/src/context.ts b/packages/ckeditor5-core/src/context.ts
index 8b116700db9..be43823a21f 100644
--- a/packages/ckeditor5-core/src/context.ts
+++ b/packages/ckeditor5-core/src/context.ts
@@ -164,12 +164,19 @@ export class Context {
 
 		if ( !translations && global.window.CKEDITOR_TRANSLATIONS ) {
 			/**
-			 * Function _translate from translation-service.ts gets translations from
+			 * When translations are not provided directly but available via global.window.CKEDITOR_TRANSLATIONS
+			 * (which can be set by CKEditorTranslationsPlugin during manual tests with --language flag,
+			 * or loaded from CDN translation files), we need to ensure the config.language.ui value is properly set.
+			 *
+			 * When translations are loaded via the global variable, the config.language.ui might be missing
+			 * or incorrect, which can cause issues with utilities that depend on the language configuration
+			 * (e.g., date formatting utilities). To fix this, we check if the configured language has matching
+			 * translations, and if not, fall back to the first available language from the global translations object.
+			 *
+			 * Note: The _translate function from translation-service.ts gets translations from
 			 * global.window.CKEDITOR_TRANSLATIONS when translations injected into Locale are empty.
-			 * Value of global.window.CKEDITOR_TRANSLATIONS is provided by CKEditorTranslationsPlugin from
-			 * ckeditor5-dev-translations package (for example in manual tests) when --language flag is used.
-			 * Function _translate is called often and has no access to the editing editor config, so here is a better place to
-			 * check if translations will be taken from dev-translations, and if yes – update config.language.ui value.
+			 * Since _translate is called often and has no access to the editor config, this is the better place
+			 * to check if translations will be taken from the global variable and update config.language.ui accordingly.
 			 */
 			const devTranslations = cloneDeep( global.window.CKEDITOR_TRANSLATIONS );
 			const uiLanguageFromConfig = typeof languageConfig === 'string' ? languageConfig : languageConfig.ui;
diff --git a/packages/ckeditor5-utils/src/config.ts b/packages/ckeditor5-utils/src/config.ts
index fea3037786f..279b14f265b 100644
--- a/packages/ckeditor5-utils/src/config.ts
+++ b/packages/ckeditor5-utils/src/config.ts
@@ -193,9 +193,9 @@ export class Config<Cfg> {
 
 		// Iterate over parts to check if currently stored configuration has proper structure.
 		for ( const part of parts ) {
-			// If target[part] is not an object or array, initialize as empty object.
-			if ( typeof target[ part ] !== 'object' || target[ part ] === null ) {
-				target[ part ] = {};
+			// If there is no object for specified part then create one.
+			if ( !isPlainObject( target[ part ] ) ) {
+				target[ part ] = Object.create( null );
 			}
 
 			// Nested object becomes a target.
@@ -206,7 +206,7 @@ export class Config<Cfg> {
 		if ( isPlainObject( value ) ) {
 			// We take care of proper config structure.
 			if ( !isPlainObject( target[ name ] ) ) {
-				target[ name ] = {};
+				target[ name ] = Object.create( null );
 			}
 
 			target = target[ name ];
@@ -218,7 +218,7 @@ export class Config<Cfg> {
 		}
 
 		// Do nothing if we are defining configuration for non empty name.
-		if ( isDefine && typeof target[ name ] !== 'undefined' ) {
+		if ( isDefine && typeof target[ name ] != 'undefined' ) {
 			return;
 		}
 

From 03fd7e66eba8d43b6783160901cadda4b5519607 Mon Sep 17 00:00:00 2001
From: l-lejman <l.lejman@cksource.com>
Date: Wed, 25 Jun 2025 19:14:29 +0200
Subject: [PATCH 5/6] Remove clone deep from translations in config

---
 packages/ckeditor5-core/src/context.ts | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/packages/ckeditor5-core/src/context.ts b/packages/ckeditor5-core/src/context.ts
index be43823a21f..2d2a959008e 100644
--- a/packages/ckeditor5-core/src/context.ts
+++ b/packages/ckeditor5-core/src/context.ts
@@ -20,7 +20,6 @@ import { PluginCollection } from './plugincollection.js';
 import { type Editor } from './editor/editor.js';
 import type { LoadedPlugins, PluginConstructor } from './plugin.js';
 import type { EditorConfig } from './editor/editorconfig.js';
-import { cloneDeep } from 'es-toolkit/compat';
 
 /**
  * Provides a common, higher-level environment for solutions that use multiple {@link module:core/editor/editor~Editor editors}
@@ -178,19 +177,19 @@ export class Context {
 			 * Since _translate is called often and has no access to the editor config, this is the better place
 			 * to check if translations will be taken from the global variable and update config.language.ui accordingly.
 			 */
-			const devTranslations = cloneDeep( global.window.CKEDITOR_TRANSLATIONS );
+			const globalTranslations = global.window.CKEDITOR_TRANSLATIONS;
 			const uiLanguageFromConfig = typeof languageConfig === 'string' ? languageConfig : languageConfig.ui;
-			const hasMatchingTranslations = devTranslations[ uiLanguageFromConfig! ];
-			const defaultDevTranslationsLanguage = Object.keys( devTranslations )[ 0 ];
+			const hasMatchingTranslations = globalTranslations[ uiLanguageFromConfig! ];
+			const defaultGlobalTranslationsLanguage = Object.keys( globalTranslations )[ 0 ];
 
 			this.locale = new Locale( {
-				uiLanguage: hasMatchingTranslations ? uiLanguageFromConfig : defaultDevTranslationsLanguage,
+				uiLanguage: hasMatchingTranslations ? uiLanguageFromConfig : defaultGlobalTranslationsLanguage,
 				contentLanguage: this.config.get( 'language.content' ),
-				translations: devTranslations
+				translations: globalTranslations
 			} );
 
-			if ( !hasMatchingTranslations && this.config.get( 'language.ui' ) !== defaultDevTranslationsLanguage ) {
-				this.config.define( 'language.ui', defaultDevTranslationsLanguage );
+			if ( !hasMatchingTranslations && this.config.get( 'language.ui' ) !== defaultGlobalTranslationsLanguage ) {
+				this.config.define( 'language.ui', defaultGlobalTranslationsLanguage );
 			}
 		} else {
 			this.locale = new Locale( {

From 1c4a0842d62e589acc3ac104a3ccc68d48481dd8 Mon Sep 17 00:00:00 2001
From: l-lejman <l.lejman@cksource.com>
Date: Wed, 25 Jun 2025 20:29:52 +0200
Subject: [PATCH 6/6] Restore clone deep to not fail unit tests.

---
 packages/ckeditor5-core/src/context.ts | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/packages/ckeditor5-core/src/context.ts b/packages/ckeditor5-core/src/context.ts
index 2d2a959008e..8ba6f37cc09 100644
--- a/packages/ckeditor5-core/src/context.ts
+++ b/packages/ckeditor5-core/src/context.ts
@@ -21,6 +21,8 @@ import { type Editor } from './editor/editor.js';
 import type { LoadedPlugins, PluginConstructor } from './plugin.js';
 import type { EditorConfig } from './editor/editorconfig.js';
 
+import { cloneDeep } from 'es-toolkit/compat';
+
 /**
  * Provides a common, higher-level environment for solutions that use multiple {@link module:core/editor/editor~Editor editors}
  * or plugins that work outside the editor. Use it instead of {@link module:core/editor/editor~Editor.create `Editor.create()`}
@@ -177,7 +179,7 @@ export class Context {
 			 * Since _translate is called often and has no access to the editor config, this is the better place
 			 * to check if translations will be taken from the global variable and update config.language.ui accordingly.
 			 */
-			const globalTranslations = global.window.CKEDITOR_TRANSLATIONS;
+			const globalTranslations = cloneDeep( global.window.CKEDITOR_TRANSLATIONS );
 			const uiLanguageFromConfig = typeof languageConfig === 'string' ? languageConfig : languageConfig.ui;
 			const hasMatchingTranslations = globalTranslations[ uiLanguageFromConfig! ];
 			const defaultGlobalTranslationsLanguage = Object.keys( globalTranslations )[ 0 ];