Translations into Tatar language ('tt' vs 'tt-cyrl' / 'tt-latn')

? This issue is unconfirmed, still to be investigated by siebrand

As i noticed some time back, new translations into Tatar language is not tranferred to tatar Wikipedia (tt.wikipedia.org).

My investigation:

1. For translators available three language code for Tatar language (through language selector in top right corner of Special:Translate) - "tt" (татарча), "tt-cyrl" (татарча) and "tt-latn" (tatarça). All of them are now contain translated messages. There are no warning messages for translators.
2. But if you go to a direct link wth "tt" langcode, you will see that
   • caption for "tt" is "татарча/tatarça"
   • warning message "This language code should remain unused. Localise in tt-cyrl please"

If "tt" is deprecated, please

1. disable this code for selection in the Special:Translate
2. change caption of this code that displayed in the languages list, to "татарча/tatarça"
3. copy all translations from "tt" into "tt-cyrl" (if target translations is not exists)

This should be applied also to meta-wiki and another Wikimedia wikis.

A separate question - which language file is used for Tatar Wikipedia (cyrillic script used by default; no working switch)? tt-cyrl.json or tt.json ? As i noticed, tt.json files now does not creates.

Note: In Meta, MW.org etc. was used "tt" code for cyrillic translations, by default.

The first is already done during phab:T51898