Improve loading of i18n message files for non supported locales

In the scope of NXP-22722, Improve loading of i18n message files for non supported Crowdin locales. Right now, overriding the messages file for Spanish es-ES requires knowing that es also needs to be overridden, because it is duplicated from es-ES.

Two types of fallback strategies should be considered to deal with this issue:
a) falling back from single-code locales (like es) to a supported double-code locale
b) falling back from a non-supported double-code locales to supported double-code

A global JS map could be used on the locale loader to handle these issues, but they may raise sync issues when contributing from other bundles. Example:

window.nuxeo.I18n.localeFallback = window.nuxeo.I18n.localeFallback || {
  'es': 'es-ES',
  'es-BO': 'es-ES'
};