native ecosystem support for language tag supersets e.g. en and en-US

[samuelstroschein]

Problem

A language subtag like en-US needs to be treated differently than a "base" language tag like en or de. A few examples:

1. Not all messages in en-US need to be translated
2. Paraglide JS needs to fall back from en-US to en if a message does not exist in en-US

Additional information

Due to different fallback algorithms, an inlang project must provide a fallback to inlang apps to ensure consistency. The worst thing that can happen is inlang app 1 and app 2 treat fallbacks differently.

• fallback settings in an inlang project
• The missing message lint rule should not report on subtags like en-US, making translations of en-US optional
• @opral/inlang-paraglide-js needs to fallback to the base language tag for supersets/subtags like en-US. A fallback logic is required for this though

[LorisSigrist]

Paraglide JS needs to fall back from en-US to en if a message does not exist in en-US

It should already be doing that, I'll check to make sure