There isn't really an "implementation" here - all that was done was the Community user's display name and bio was manually edited by someone on those sites. It was simply never brought up on the other two sites, so it was never done. That's not really a bug, just a lack of people caring in those places.
I've changed it on the Russian Language site to match ...
This is already possible. You aren't required to use the same username on all sites; you can use different usernames on different sites.
Simply enter a different username on a given site's profile editor, and click the translation of "Save changes just for this community". You'll have a different username for that community.
I understand that this is a bit ...
I checked on the traducir.win, the tool we use to translate strings, and found that there are two almost similar strings that say
If you need anything else, please visit our contact form and select Other for additional help.
Only one of them was translated. I copied the translation and submitted it for Japanese and Portuguese languages. The translation ...
It's already translated in Traducir, string https://ru.traducir.win/strings/13674.
We have topic about it on ruSO: Нет перевода текста в комментариях при закрытии вопроса как дубликат.
You will see the translated string, when a new revision is released (last one was rev 2019.12.20.35703).
The text for the notices for migrations when the source/destination language is different have been changed to no longer pull anything from the Db, and are now fully translatable. For migration notices between sites with the same language code the notice is the same.