Add support for advanced locale definition

[teolemon]

What

• Crowdin writes some 3 letter locales that are not recognized
• We have the possibility to rewrite those locales, but it's very dirty and may cause namespace clashes

  languages_mapping:
    two_letters_code:
      pt-BR: br
      pt-PT: pt
      zh-CN: zh_CN
      zh-HK: zh_HK
      en-GB: en_GB
      en-AU: en_AU
      en-US: en_US
      nl-BE: nl_BE
      nl: nl_NL
      zh-TW: zh_TW
      sr-CS: sr_CS
      sr: sr_RS
      ku : kmr_TR
      kmr: kmr

• The right way to do it seems to implement https://docs.flutter.dev/ui/accessibility-and-localization/internationalization#advanced-locale-definition and possibly move to fr_FR format ?
• What do you think ?

[teolemon]

Duplicate of https://github.com/openfoodfacts/smooth-app/issues/1638