I mistook to select Ukrainian. how delete it?
In the settings of your account.
Note: this discussion would be, IMO, better located in the “Troubleshooting" forum? Which is the place meant for such discussion.
To relocate it (\/!\ please, don’t create a new one \/!\), edit your original post then select said forum in the drop-down menu on top-left of the edition area.
Have a look to this short guide to help you knowing where to post your (future) discussions on Duolingo’s forums.