+1, for Ukrainian Wikipedia should definitely be supported. While there is a Wikisource in Ukrainian (and a very active one too!), the kinds of texts it contains are often badly suitable for what Luthor is trying to achieve. For example many texts are entries from public domain encyclopaedias or dictionaries, or they are literature that uses orthographies different from the current one. Neither of those factors disqualifies sentences from being used as examples strictly speaking, but it makes them suboptimal for that. On the other hand I am sure that Ukrainian Wikipedia would contain many more sentences that are a better fit.

Copyright has to be considered, as while Wikisource mostly contains public domain works, that are totally compatible with CC0 terms of Wikidata, Wikipedia is licensed under CC BY-SA 4.0 at the moment, that said it is likely not a factor for single sentences, unless someone abuses the tool to add the whole paragraphs as an example (which I am not sure even is possible technically on Wikidata side of things). If someone does abuse that, one should remember though that they can also do that without using Luthor so it is further not a factor, but I think it should have been mentioned explicitly.

Interestingly even Cambridge's website seems to be using Wikipedia for example sentences, see for example https://dictionary.cambridge.org/dictionary/english/withal