You do, but, when you're in a context where you're speaking a language, you're also thinking in that language. I wouldn't be surprised if that has an influence on one's thought processes.

Concrete example: I have less of a tendency toward hyperbole when I'm speaking my 2nd language. The reason why is simple: I don't know many hyperbolic idioms in that language, so I lack the ability to express myself hyperbolically. But I think it goes a step further: Lacking the vocabulary, I have a hard time even having hyperbolic thoughts when I'm thinking in that language.

(That said, I don't want to suggest that this is somehow an aspect of the language itself. It seems more likely that native speakers have about the same range of expression as native speakers of any other language, and I'm just lacking it because of my limited familiarity.)