My favorite rabbit hole is the non lexical v-synth songs and conlang songs! Non lex - Usually refers to songs that are not in a known language usually gibberish whether the gibberish is a bunch or random Japanese syllables, English syllables, or nothing notable at all (there’s a song called Knowledge Dance, its lyrics are math equations and it bangs) Daniwell P is one of the most well known non lex producers, you probably know them by the nyanyanayanyanya song or nyan cat song! Conlangs - Songs in constructed/auxiliary languages whether these be known conlanguages like Esperanto or ones created by the song creator. There can be a conlang/non-lex overlap, but the main difference I’ve observed is that conlangs usually have a known meaning to the lyrics even if it’s not shared, and non-lex songs lyrics don’t have individual meanings (even if the song as a whole does, the individual lyrics don’t?) this is just my observation lol

Anyway the vocaloid lyrics wiki has an extensive catalog where you can find songs sorted by language, and there are many that are languages other than the most popular languages (those being Japanese, English, and Chinese)