This thread has me wanting to take French immersion. A few years ago, I was taking courses at the Alliance Française in Toronto, but they were the usual once-a-week classroom courses and it didn’t stick. Last year, though, I took that Japanese immersion course and really enjoyed it. But even though it was far more than classroom grammar instruction, still, we were in a city where the common language is English, and it was quickly drowned out by things as simple as the conversations around me going home on the subway.
To really do immersion, you need to be in a place where the common language is the language you are learning. You need the language around you at all times–the books you read, the movies you watch, the music you listen to. A quick google led me to the University of Western Ontario’s summer French immersion program in Québec. Six weeks doing nothing but learning French, in a town that only speaks French. You sign a pledge to only speak French.
Drastic, but consider this: children learning their first languages are already in immersion. They are spending every hour of every day learning their mother tongues, along with everything else they are learning. This is why kids can go to immersion school in a language other than the ‘street’ language of their town, and not lose the street language: the world outside school keeps teaching them the street language as the world inside the school teaches them the immersion language.
Even a second language of limited fluency can get you places and help you avoid problems.
Example: back in 2007 I went to a weekend get-together in Montréal. It was in Esperanto, and I arranged to stay at a friend of a friend’s. The household spoke French, and only my acquaintance spoke Esperanto. I had nowhere to speak English. The next year, there was a conference, again in Montréal, again in Esperanto. It was held at the Université du Québec à Montréal, which is a French-speaking institution. Again, English was out of the picture.
I was able to go all over Montréal with people speaking Esperanto. I got to go off the tourist trail. I met people who I suspect were cultural activists and somewhat ill-disposed towards English-speakers from the rest of Canada–especially ones from Toronto*–but as long as I didn’t speak Englsh, I was okay. The one time we did speak English on the street, we got some snarky remarks from passers-by.
Now, this is all bound up with the effects of the past fifty years in Québec/Canada politics–google “Quiet Revolution” if you want more–but there is no denying that if you speak more than one language, you can avoid problems in one language by using another.
[sub]* After the Parti Quebecois took power in '76, a lot of English-speaking people and companies left Montréal for Toronto. It was around this time that Toronto began its rapid growth to become bigger than Montréal**–before then, Montréal was the largest and richest city in Canada.
**But Montréal still has a better-looking metro. 
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