LibreChat/client/src/localization/Translation.tsx

import English from './languages/Eng';
import Chinese from './languages/Zh';
import Italian from './languages/It';
import Portuguese from './languages/Br';
import Spanish from './languages/Es';
// === import additional language files here === //

// New method on String allow using "{\d}" placeholder for
// loading value dynamically.
interface String {
  format(...replacements: string[]): string;
}

if (!String.prototype.format) {
  String.prototype.format = function () {
    var args = arguments;
    return this.replace(/{(\d+)}/g, function (match, number) {
      return typeof args[number] != 'undefined' ? args[number] : match;
    });
  };
}

// input: language code in string
// returns an object of translated strings in the language
export const getTranslations = (langCode: string) => {
  if (langCode === 'en') {
    return English;
  }
  if (langCode === 'cn') {
    return Chinese;
  }
  if (langCode === 'it') {
    return Italian;
  }
  if (langCode === 'br') {
    return Portuguese;
  }
  if (langCode === 'es') {
    return Spanish;
  }
  // === add conditionals here for additional languages here === //
  return English; // default to English
};

// input: language code in string & phrase key in string
// returns an corresponding phrase value in string
export const localize = (langCode: string, phraseKey: string, ...values: string[]) => {
  const lang = getTranslations(langCode);
  if (phraseKey in lang) {
    return lang[phraseKey].format(...values);
  }

  if (phraseKey in English) {
    // Fall back logic to cover untranslated phrases
    return English[phraseKey].format(...values);
  }

  // In case the key is not defined, return empty instead of throw errors.
  return '';
};