I don't speak Japanese

a first language, a mother tongue, a native tongue, a native language

(linguistics, uncommon) a proto-language, a mother language (from which later daughter languages evolve)

the Japanese language

(chiefly women's speech) first person singular pronoun: I, me

(polite) first-person singular pronoun: I, me

a personal or private matter / (by extension) selfishness

(formal) first-person singular pronoun: I, me

personal, private / secret

a personal or private matter / selfishness / privacy, secrecy / (women's speech, obsolete) a brother-in-law