Configuration options for locale<->language code imports/exports

[stevenle]

Description

Some sites use locales that have a combination of language code and country code (e.g. en_CA, fr_CA) but need to import/export based on language codes only (e.g. en-GB and es-419) as determined by the localization tool used.

We should have some configuration for localizations that a particular locale code imports/exports translations from a particular lang code.

e.g.

i18n: {
  locales: ['en_CA', 'fr_CA'],
  localeTranslationAliases: {
    'en_CA': 'en-GB',
    'fr_CA': 'fr-CA',
  },
}

(configuration name not final)

This would tell the system "when exporting to google sheets or another localization system, use "en-GB" for en_CA and "fr-CA" for fr_CA".