I have the same thing! I'm Dutch, but I also talk and chat a lot in English. My Mantra's are Dutch personally, though I think they would also work in English. Half of the time I think in English anyway. I even noticed I dream in English a lot. So I would say, whatever feels right for you.

Though I'm not sure xD I'm interested to know if anyone ever tried out a Mantra that's not in their Native language.