I'm a native English speaker and relatively fluent Japanese speaker. I used to speak only Japanese for several days straight, dreamed in Japanese, thought in Japnese etc. when I lived there.

There are words in Japanese I cannot translate to English - any translation just has the wrong meaning outside of the Japanese context.

My friends who've seen me speak in both languages say I have quite different personalities in each language. I would say that it's primarily related to the cultural context I learned the words in.

In the same way different groups of friends bring out different aspects of me, so too do different languages.