There seem to only be
2 songs tagged as Welsh on vocadb. I don't know any Welsh, but the only languages commercial synths are capable without editing phonemes like crazy are Japanese, English, Chinese, Korean, and Spanish.
Edit: There may be a few UTAU voice banks capable of the language, though! I think there are some UTAU wikis you might be able to look up?
This is just my personal experience, but... I speak Japanese and even though there are tons of Japanese vocal synth songs, I really do not recommend listening to vocal synth songs as a major thing on your playlist (especially for beginners) because they are harder to understand than humans.
What I do recommend is making a playlist and browsing forums/YouTube to gradually add songs in your target language to so you can enjoy listening/singing along while getting used to the pronunciation/pick up words from songs with repetitive lyrics. Find out what genres are popular in the language and hopefully some of them suit your tastes, maybe consider some folk songs. (I can not recommend any songs as I do not know any, though, hopefully someone can recommend something.)