Use ISO-639-conforming language codes for per-toot-language interface

[Wuzzy2]

Pitch

(This is a follow-up of #17073.)

Currently, the recently-added per-toot language UI uses non-standard language codes. It's in all-capes. However, ISO-639, the standard for language codes uses lower-case. For example, it shows "EN" for English although "en" would be correct.

I'm not sure which standard is used for the language codes currently shown in the UI, but if I would guess, it's literally ISO-639, but converted to upper-case for some reason.

Motivation

Conforming to ISO-639 just makes sense to me. If uppercase was chosen because "it looks better", that's a poor argument IMHO. The ISO standard is case-sensitive. Also note that there is another popular ISO standard for country codes, and it is in upper case, so this is definitely a source of confusion.

Just sticking to ISO-639 and not do some arbitrary modifications to it reduces confusion.

[Wuzzy2]

Since that was closed as completed, may I ask which commit(s) contain(s) this change?

[nightpool]

The idea that "Upper case means country code and lower case means language code" appears to be a complete invention with no basis in actual usage or specification text. The drop-down uses the full native names of the languages, there is no confusion here between language code and country code.

[Wuzzy2]

So apparently, ISO recommends to use lower-case but they also say these codes are case-insensitive.

https://www.loc.gov/standards/iso639-2/faq.html#21

So I guess whether to choose lower or upper case is a matter of taste then, I guess?

[nightpool]

(note: Github now distinguishes between issues closed as "completed" and issues closed as "not planned". This is a very new feature, and I'm not sure it's an entirely useful one, but I've adjusted this issue's "close type" to match the correct disposition of this issue)

[mro]

@Wuzzy2 thanks for the thorough research at ISO. It's a pity they don't make distinct writing for country and language mandatory.

[mro]

a matter of taste then

I would prefer a reason why to go against the ISO recommendation.