For me, the appeal lies in being able to sustain multiple personalities without being certified and thrown into an asylum. It's the same kind of appeal as role playing. Especially if you are good enough in more than one language that your sense of language gets relativized so you don't have a psychological "home" language. This has practical advantages too. You can pick a language to process given thoughts in, in your head, but use another for input/output. You can also talk in code when others are listening, with others who share a minority language in the room with you. I sometimes do the former, but I don't do the latter very often. But I've overhead people doing it, and it's very interesting to listen to what they choose to process in (say) Hindi vs. English.

You may be vastly more fluent in a one of your languages and do 90% of your thinking/communication in it, but you'll still be in a sort of stepped-back meta-state with respect to it, because there will be a dim awareness in the back of your mind that there are entirely different ways to think what you happen to be thinking. Any jump from 1 to 2 throws the human brain into inductive generalization mode, and nudges it towards a new level of generalized thinking.

It's like owning a dual-boot computer, the kind where you pick Windows/Linux at the start of the boot sequence. Or in my case, a triple-boot (English/Hindi/Kannada though Kannada in my case lacks drivers for the keyboard and monitor, since I cannot read or write it, only speak/understand). You can even run emulators for one language while being booted up in another.

Languages are social operating systems. I find that there are thoughts you can think in one language that you simply cannot in others. If you are fluent, your whole personality may change, from the pitch of your voice to whether you are gentle or rude, etc.

Partly it is a intrinsic features of a language. Nobody believes the Sapir Whorf hypothesis in its naive form any more (
http://en.wikipedia.org/wiki/Lin...) but in a more nuanced modern form, there's plenty of work on how thinking changes with language. The apocryphal "30 words for snow" effect is real.

But partly it is also a use-case thing. Just like you might hack in Linux and do Word/Excel work in Windows, you do different things with different languages. What you typically do with a given language shapes your personality while within that language. Again the dual-boot analogy helps. If you are a command-line hacker but hate the bureaucratic fussiness of the Windows environment, you may feel powerful and godly while in Linux mode and frustrated, whiny and annoyed in Windows mode.