Do people think only through their mother tougue?

I think only through my brain.

I grew up speaking Russian, with a heavy Ukrainian dialect/accent mind you, and learned American English from five years of age. I'm fluent in both, I slip up in both (who doesn't), but I think through English. Fancy that.
I suppose for me it's because of where I live. If I'm thinking of something I'd like to tell my friend, I'd like to tell them in English and not run off on Russian. So, probably just what that person is comfortable with.

Cheers,
Алиса

People think in the language(s) they feel comfortable. To be honest I have more difficulty speaking in Portuguese (Native) than the other languages I use on a daily basis (English at work and Japanese at home).

@Alisa: How does Russian sound with an Ukranian accent? Wouldn't it be easier just speak in Ukranian? Or are the languages unintelligible?

спасиба!!