Guess that depends on whether you mean "having fully mastered the language" (technically impossible for anyone, even natives, in any language, since you could argue that you can never really learn everything there is to learn in a language) or "knows most of it pretty well" when you say "knows Japanese". I'm sure there are a couple of good examples of the latter here on BL. Like Kotonoha. Also, ossan is native Japanese, so yeah...