Thanks for answer!
Finally, I reached out that the translations had to be removed from tribe-events-community domain “add” strings and translate all English-language(lisää to add etc). Cyrillic alphabet and accented in brauser address bar is not supported.
mydomain/events/community/добавлять?lang=ru – not working
mydomain/events/community/lisää?lang=fi – not working
I’m not yet 100% sure, but it seems to works that way:
mydomain/events/community/add?lang=ru – working now
mydomain/events/community/add?lang=fi – working now