I’m talkin you wake up, and you can speak, read, and write in another language PERFECTLY. So perfect that you could get UN certified.
So which language would you pick, and why?
PS: Don’t wimp out and say English, that’s a cop out.
I would pick a language over In My Humble Opinion. Here I’d phrase the question a little differently.
Okay, then, I’ll go first 
And here is where I cop out and say it’s a toss-up
Maybe Maori (indigenous language of New Zealand), as it is about to die out.
Ditto for Navajo.
On the other hand, I could probably make big money just interpreting if I knew Japanese.
That said, I probably would choose Navajo. It’s in the greatest danger of dying out. At least Maori is still taught in New Zealand schools, as far as I know. I imagine the only place Navajo is still spoken is well, on reservations.
I wish I could learn every dying language out there. I hate seeing languages die out 
GQ if for questions with factual answers. Polls and surveys go in IMHO.
Ok, Mods, move this if I’ve gone and stuck it in the wrong place.
The language of love.
Once you are fluent in that, does anything else matter?
Fluent Dutch or German would do wonders for my career (i.e. it would make that dissertation come along a lot faster). I’d probably go with German as it’s more difficult for me than Dutch so I could use the leg up.
Or LATIN! Of ALL the damn languages that are endlessly complicated! I just don’t have the patience to spend any more time on it-- I feel like I get nowhere.
Chinese wouldn’t be bad, either.
Japanese. It seems like the hardest one that would be moderately useful. I have hope of learning other languages – Japanese, not so much.
Japanese. That way, I wouldn’t have to wait for subtitled versions of anime to be released here in the US.
American Sign Language.
I’ve already lost one job opportunity because they were looking for an audiologist fluent in ASL.
I’d like to learn either Russian or Welsh, simply for the reason that I think they sound cool. Spanish, however, would probably be more useful.
Tough decision. I’d say… err… Sanskrit. Fruity new-age spiritualism and ‘unique resonance’ aside, it’s a very fascinating language and sounds rather nice as well. But unfortunately, it’s not of much use these days - you said ‘UN certified’, and I’m pretty sure the UN has no need of interpreters between English and Sanskrit.
If I had to choose a language that has signifigant demand for interpreters, I’d choose Arabic.
If you could change the word speak to express, I’d like to be able to fluently know American Sign language.
Otherwise I think it would be interesting to learn to speak Cherokee.
German. Only because I spent a year taking German and feel like I wasted my time. I can’t really remember anything and I’m never going to get that year back. I’d like to feel better about that year.
I can remember how to say one thing:
Ich sollte meine Eltern anrufen.
I shoudl have called my parents.
Japanese. I’d have pretty writing and the ability to read manga without getting it translated first. (Such a hassle…I found yet another title just yesterday that sounded really interesting but I couldn’t get translations for)
My second choice would be French because I’ve already spent three years studying it and it would be awfully nice to wake up and be fluent.
jessica
Hindi. Maybe Irish Gaelic. Or Mandarin Chinese. Ooooh, Nahuatl! Or Tok Pisin. No wait, Hawai’ian! Perhaps Quechua. Or !Kung, gotta love the African languages with all the clicks. Maybe Malayalam or Bahasa Indonesia. Korean and Japanese are nice. Ainu would be neat. That or Tibetan. Or Haitian Creole. Or Mohawk. Turkish and Finnish are beautiful languages. As is Persian. And Inuit.
Whaddya mean I can only choose one?
mandarin chinese, I love to listen to crouching tiger hidden dragon in chinese. It would be cooler if I knew what they were saying 
Around here (Texas), Spanish would be very useful. I never could get the hang of it in school.
I would really love to learn a Native American language, maybe one of those that is close to being lost forever.
Italian just because a bunch of my family still lives there and it’d be useful if I ever visted them. Other than that I really don’t care about any other language.(Yes, I guess I’m one of those “Xenophobic” americans.)
Not really sure. Japanese would be useful for me, but I feel like I’ve been studying it long enough that I’d be wasting my wish to use on a language I already half-knew.
Probably Chinese (Mandarin), but maybe Arabic or Hindi.