Mother tongues are special. Generally immigrants will do better at foreign language(s), if also getting proper education in their mother tongue. There's also been studies with brain scans showing that even "bilingual" people usually have one mother tongue, and treat all other languages as "foreign" (separate parts of the brain deal with the two "groups" of languages).

On a more anecdotal level, I had a funny experience when I went to Japan -- I was somewhat passable in French before I went, and after about half a year of being in Japan (before I had really become fluent in Japanese) I tried speaking some simple French with a French teacher. It was impossible -- every sentence started in French, and ended in Japanese :) After I got better at Japanese, my French started coming back too -- but I'm still hoping to spend a month or two in France to really get it up to a usable level. Interestingly, I didn't have any problems with my English while there -- probably because I was already fluent in English (my mother tongue is Norwegian).