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 #1638