I really object to the claim that your first language will always be stronger. Just last week, I saw a TV program about people who migrated from the Netherlands to an English speaking country more than 40 years ago. Some of them could still speak Dutch fluently, but there were also those who could no longer and were much more comfortable speaking English.

I learned English in school, but I only became fluent later on in life. When I was around 30, I worked in an English speaking environment and also at home I spoke English. I was thinking in English and there were days that I did was surprised when I heard some Dutch being spoken, suddenly realizing I had been thinking in English all the time. I felt at some time that the two language become one in my mind, that I could effortless switch from on to the other. I also acquired the ability to simultaniously translate from English to Dutch. I even sometimes when I was doing this, would find my mind to wander off, as if it had become a purely automated task. The surprising things is that I am a dyslectic and often have to look up the spelling of words. Dutch spelling seems to give me more trouble than English.

I think that many bilingual people share the same experience, but that it is hard to believe for someone who is not bilingual her/himself to believe that a second (or third) acquired language can be as intergrated as your mother tongue.