So, I came up with a new way to say "I speak language x". (By new, I mean I first thought of the idea, then realised that I once saw the same thing happen in Esperanto in "Mi parolas Esperante.") It may have horrible implications or it may work really well, but oh well, I'll see what you guys think.
Basically, the idea is that the language is a kind of unofficial adverb. Here are examples:
mi toki Inli - I speak in an "Inli" manner - I speak English
mi toki Nijon - I speak in a "Nijon" manner - I speak Japanese
Or, a longer version that makes it extremely clear that the adverb is a language:
mi toki pi toki Inli
mi toki pi toki Nijon
nasin ni li pona anu ike tawa sina mute?