Tmnt 2012 hc:
I think that if the turtles were humanized they would all be black and/or blasian since they’re siblings and would have to have the same genetic tree (in 2012 I’m pretty sure they’re biologically related). However I love idea that they have slight differences between them since they hang out in different areas in nyc (a diverse city with a bunch of different cultures that I think would really interest them).
So anyways I hc that Splinter brought them all up to be bilingual, however Raph and Donnie aren’t as good at Japanese anymore since I think they aren’t as good with languages and they would consume a lot of English content growing up + Leo probably started translating Splinter for them and they kinda stopped needing to use it, so it faded a bit. I think Raph stopped speaking Japanese but can still understand it and could speak it (with some grammar and pronunciation errors) in a pinch. Donnie rarely speaks Japanese but he speaks it from time to time with Splinter, Leo, or Mikey. Leo is probably fluent in Japanese and English and I think her (Leo is transfem in my books) and Mikey can read Japanese while Raph and Donnie can’t. However, I think that Leo code switches and sometimes makes tiny grammar/pronunciation errors, with Mikey being the best at Japanese of all their (Mikey is aroace gender-fluid) siblings.
I don’t think Raph is a big language buff (unlabeled he/him non binary Raph supremacy), so he probably just knows English and Japanese. Meanwhile, I think Donnie knows code languages and probably taught themself (They/them for Donnie, source: me) obscure/dead/constructed languages like Esperanto or Latin. Leo probably can understand/read Spanish to some extent, though she’s barely conversational speaking it. She might also know certain phrases in a bunch of different languages since I hc that she wants to travel. Mikey, in my opinion, will be the best at picking up languages, probably fluent in Latin American Spanish, Mandarin Chinese, and Cantonese, conversational in Italian and Korean, and has begun to pick up some Haitian Creole (though her pronunciation isn’t good because she probably sounds either very American or has WAY to heavy of an accent). They also codeswitch and mesh together languages pretty often, so his siblings have begun to pick up a bit of their languages, especially Leo (how she learned Latin American Spanish)
I think they speak primarily English among themselves and Japanese/English with Splinter, who is plays a little game with himself when he hears music to guess the language it’s in