While linguistics experts are reluctant to talk of a 'third language' being formed in the brain of an immigrant, studies are now beginning to show that the brain does find it difficult to completely compartmentalize two distinct languages without merging them in subtle ways, says U of T linguistics professor Ana Teresa Pérez-Leroux.
What we are finding is that we don't and can't have complete separation between different languages in our heads. Yes, you can become very talented with your acquired language but there will always be a kind of window in our brains where one language will always 'leak' into another.
For example, a fluently bilingual speaker may say something in almost perfect English with the exception of one or two words or word structures from their mother tongue infiltrating the sentence. One instance is a person whose native language is German and who has mastered the English language saying something like, "I to the dining room go.
My own experience is that this language merge isn't always so subtle, especially amongst the less educated working class immigrants. As regular readers know, I'm an immigrant, but I learned my second language as a child going through school. I was always very aware of the struggles with English that my parents' generation of Finns and other immigrants experienced. Something very interesting happened to many of the Finns (and I believe this happens in other languages too) - they developed amongst themselves what became called "finglish", a mixture of English and Finnish. They would take an English word and add a Finnish ending to it, usually a vowel. For someone who was unfamiliar with it, it sounded hilarious and puzzling. Long ago, I bookmarked an actual article about finglish as practised in the US. Some of the examples given are unfamiliar to me so they must be locally variable.
One time we were visiting with some family in Finland, at the same time as some other older relatives from Canada happened to also be there. My young Finnish cousin, who knew English fluently, was listening to their speech with a puzzled look on his face. He commented to me later that we was amazed that they could not speak English correctly nor could they speak Finnish properly either! Funny yet sad.