[Coda]

It's pretty true. I've known foreign exchange students whose internal monologues had switched to English. And when I was still in practice, I would think in Japanese pretty often.

That said, dreaming in a language doesn't mean you're completely fluent, just that you've internalized the language instead of thinking of it as foreign.

As for practicing... Bafflebox and I used to hold entire conversations in Japanese, and sometimes we'd even get pretty philosophical about stuff doing so. Dictionaries help. XD

Edit: Just for fun.

さ、本当だよ。思いは英語になった留学生を知ってた。そして、練習の時に、僕もよく日本語で思 った。

それでも、ほかの語で夢を見ても、完璧に上手意味じゃない。外国語じゃないと思ってなっただけ だ。

(Literally: "Well, it's true. I knew exchange students whose thoughts had become English. And, when I was practicing, I would often think in Japanese. Nevertheless, even if you dream in another language, it doesn't mean that you're perfectly skilled. It's just that you started thinking that it wasn't a foreign language." That last sentence is really awkward to translate; the verb phrase I used more literally means "it became that you think".)