AFAIK, interface can only be English.
At least, that comes from information that I had gathered before. Please, look at that chapter in "Stack Exchange" article in Ukrainian Wikipedia (with references), if you understand Ukrainian. In short:
- At first Stack Exchange allowed creation of non-English sites (this is about both questions/answers and user interface).
- In 2015 creation of non-English sites was stopped; localized sites are now allowed only for those languages, that already have localized Stack Overflow's (i.e. pt, ja, ru, es); and no new localized Stack Overflow's are going to be created in the nearest time.
- Language-learning sites are special case in that they allow questions and answers in target language (not just quotes within questions/answers, but questions/answers themselves too); this is reasoned by full immersion for learners; but they still are considered as English-language sites.
Maybe something changed since then (I have no relation to Stack Exchange administration and cannot guarantee that my info is up-to-date). But my perception is that creation of "Ukrainian Language" is allowed only as far as it's considered as English site for learning Ukrainian language. Ukrainians can benefit from it too, as questions/answers in Ukrainian are allowed (and I suppose most of questions and answers will be in Ukrainian), but the formal limitations remain.
P.S.: Ukrainian Wikipedia article contains references. Please tell me if I should copy them here for your convenience. Maybe, we should ask some administrator directly, but I don't have any special hopes for that.