I couldn't imagine learning a language for "fun", but I know others do. I can easily imagine a lot of them dropping-out and giving up, once they've either learned enough to be useful or decided it is not for them.
But me? I've moved from Scotland to Finland. I'm very slowly learning the language. I don't expect I'll ever be fluent, but I need to try. People who move abroad, and avoid learning the language are not so well respected, and rightly so.
Thus far I've managed to have medical treatment, buy a flat, and do lots of complex things in English, but it would still be better for me to learn Finnish. If only so people don't need to switch languages, solely for my benefit, at parties, etc. (That's appreciated, but also a constant reminder of how much I suck!)
IMO, you have to have a reason for the language. Mine is that I wanted to read and watch anime/manga/tv/books/games that weren't being translated. Of course, these days they're all being translated faster than I can read them. But it was enough to get started, and I definitely enjoy them more than if I didn't know the original Japanese, too.
But I've tried to learn other languages that I don't have a use for, and I've found it nearly impossible. There just isn't enough motivation.