How do you choose which language to learn?

Interesting indeed. Were those the only two reasons given though?

either for work

From my studies in linguistics, those who used this reason to study a second language were the least successful at doing so.

Post 9,
Although not your intent, your comment may stir some controversy. You wrote that:

" Some times that means that they don't feel themselves in their own culture "??

This assumption in my experience is way off target, maybe even an insult to multilingual individuals. Taken your statement one step further one could say it is complementary to those who never learned another language.

I have worked professionally in other western countries and those coworkers least admired were those who never tried or considered worthwhile learning the basics in the visiting country. And here goes my own controversial statement, those individual's mother tongue was usually English.

because they want to adopt another culture. Sometimes that means that they don't feel themselves in their own culture.

I don't think that's necessarily insulting, and there may be some truth to the statement. A lot of people learn a language so that they can talk to people who speak it, travel in a country where it's spoken, read books in it, etc. These are all ways of learning more about the culture, and no doubt there are people who go so far as to try to adopt a different culture (or elements of it) because they like something about it. That may or may not have to do with dissatisfaction with the adoptor's own culture. #9 did say "Sometimes."

Post 5,

While traveling in Europe over the years my wife in three occasions needed a doctor....perhaps only by chance, but all three times the private doctors spoke English. It happened recently in Roses, Spain , in Austria six years ago in a very small skiing town near Innsbruck and last year in Paris near Rue Cler street. In all three cases we stopped by the office and waited a few minutes , no appointments . Yes, had they not spoken English we would have asked which doctor did!

Adopting, yes. Uncomfortable, no. (Maybe not, or am I in denial? ;)

Anyway, the book sounds intriguing....