You can extract the game's data files (*.mnf) with this tool:
https://www.esoui.com/downloads/info...okBrowser.html
You need to point it to the install directory not the c:\users\...\documents\elder scrolls online folder!
I think the files contain the text sources which need to be translated for custom languages.
Don't ask me please what else needs to be taken into account. Somehow I'd guess you need a 2character ISO code for your language, should be "UA" I think?
And then you need to create an "AddOn" which contains the new translation files and implements them somehow to be used for your custom language.
Maybe Sharlikran is able to help here how this works.
He has helped a guy who wanted to add "Turkish" as custom language here e.g.
https://www.esoui.com/forums/showthread.php?t=10132
Maybe it provides you additional info.
General localization support information can be found here (but I guess this will not help you in detail as it's meant to be used for addons):
https://wiki.esoui.com/How_to_add_localization_support
Maybe have a look at the other mentioned "custom languages" at that link, how they did it.