English you can do stuff like jazz standards and other old royalty free music, nursery rhymes are good for any language. Searching for free "royalty free" music with vocals can get you some stuff too. For jp I search on dova syndrome which has a lot of free stuff but it helps if you're at least familiar with Japanese, because you may have to transcribe lyrics from PVs or by ear, they aren't always provided.
To save you some headache, check out:
(this should take you to the tag for songs with vocals, bc it's kinda difficult to figure out)
(imho this person is one of the most vocaloid-like so it might be a bit easier to learn their works)
Be sure to triple check the rules for using these, but have fun!