Proper Internationalization(i18n) Support

[gk-wino]

Summary
Fastest-Validator currently doesn’t provide a clean or native way to define internationalized validation messages and dynamically switch between locales based on a locale option. As a result, applications that need multi-language support must implement custom workarounds, message maps, or manual message overrides, which becomes messy and difficult to maintain.

Problem

• Validation messages are static and tied directly to rule definitions.
• There is no built-in mechanism to specify message strings per locale.
• There is no global or per-validation locale option that automatically selects the correct localized message.
• Implementing i18n requires writing custom logic that defeats the purpose of using a structured validation library.

What’s Needed

1. Ability to define messages in a multi-locale format, for example:

   messages: {
     required: {
       en: "The field '{field}' is required.",
       fr: "Le champ '{field}' est obligatoire.",
       sw: "Sehemu ya '{field}' inahitajika."
     }
   }

2. A validator-level or call-level locale option:

   const validate = v.compile(schema, { locale: "fr" });

   or:

   v.validate(data, schema, { locale: "fr" });

3. Automatic fallback (e.g., fallback to en if the specified locale is missing).

Why It Matters
Proper i18n support is essential for real-world applications that serve users across different regions. Without it, developers are forced to duplicate schemas or build their own localization systems around the validator.

Proposed Outcome
A first-class internationalization API that allows developers to define localized messages cleanly and select the active locale at runtime.