[6.x] Improve list of CP's formatting locales

[joshuablum]

Someone reached out via support saying their preferred formatting locale is missing. The currently available locales only listed the Control Panel's translation languages, so regional variants like fr-CH were missing. It's actually possible to type in any value and save, not just the ones offered in the list. However, nothing in the UI said so.

This PR:

• Adds de-AT, en-GB, es-419 (TIL that's Latin American Spanish's code), fr-CA, fr-CH, and it-CH. Each one formats dates or numbers differently from its base language (e.g. fr-CH → 06.01.2026, it-CH → 1’234’567.89 (some wild thousands separator)).
• Adds the user's browser languages (navigator.languages) to the options, so their own locale shows up without being added to the list explicitly.
• Shows the region in labels ("French (Switzerland)" instead of "French") and sorts options by label.
• Makes options searchable by locale code as well as by name.
• Updates the instructions to say that any locale code can be typed in.

Browsers don't provide a way to list every locale they support, so the list stays curated. Anything else can still be typed in, and the browser checks it as before.