[Input Sources] Multiple input sources for same language use the same abbreviation

[git-f0x]

E.g. Serbian and Serbian (Latin) both show "rs", so it's impossible to tell what input source is currently used at a glance.
GNOME adds an index to the abbreviation (e.g. rs₁ and rs₂).
Unrelated, but the abbreviations aren't the same as in the design issue. For English (US), "us" is used instead of "en", but it might be better as is to better distinguish between US and UK keyboard layouts.