Add (server-side) support for language variants (like Simplified/Traditional Chinese and PT and BR Portuguese)

[teolemon]

What

• Add support for language variants (like Simplified/Traditional Chinese and PT and BR Portuguese)
  • see Add support for Traditional Chinese #810
  • Languages with variants are not supported smooth-app#1638
• Quick fix: creating a fake locale like xt, xs… and adapting Smoothie to use it
• Long fix: supporting true pt_BR, zh_CN, zh_TW langcode

Part of

• 🌍 Make Open Food Facts work well in all countries in the world #5690

[github-actions]

This issue has been open 90 days with no activity. Can you give it a little love by linking it to a parent issue, adding relevant labels and projets, creating a mockup if applicable, adding code pointers from https://github.com/openfoodfacts/openfoodfacts-server/blob/main/.github/labeler.yml, giving it a priority, editing the original issue to have a more comprehensive description… Thank you very much for your contribution to 🍊 Open Food Facts