I lived and worked in France for two years in my early twenties and really made an effort to make good friendships with local French and try to adapt to and understand the culture. Once in awhile people would hear me speak to people back home in Canada on the phone and on several different occasions they commented to me that I speak much more loudly and boisterously in English. It was very unconscious to me.

In short, yes, speaking a different language can very much affect your personality. But it is not just the language, it is the culture that you express that language within, and also how the structure and vocabulary of that language itself reflects the culture in which it is used.

My favorite thing in France was when I would learn some new word or concept and I would "get" how to use it in French, but I couldn't fully and perfectly translate it to English because of how it just doesn't work quite the same within our culture.

e.g.: "C'est normal." It seems like an easy translation, but it requires a real understanding of the differences between French and English/American cultural views of conformity to really be able to see how it doesn't translate quite correctly. It means sort of "how things should be" or "as expected", but even that isn't quite correct, because the "should" is so implicit and assumed in a way that we don't assume in English.