Esperanto. The wave of the future.

The is a great case study in how all tastes and preferences are subjective (i.e., there is obviously no correct answer).

I would still recommend German, if only because it gives the best insight into the structure of English. The two languages have both enough in common and enough that is different so as to make studying German grammar VERY useful in gaining a greater appreciation of English grammar. That benefit outweighs (IMHO) any guesstimate as to which language will be the "hot" language in "Year t+n".

P.S. Latin would also qualify under this paradigm, but it wasn't on your list.

"Naturally I am biased in favour of boys learning English; and then I would let the clever learn Latin as an honour and Greek as a treat. But the only thing I would whip them for is not knowing English. I would whip them hard for that."
--Winston Churchill

German. In the last few years I have spent 2 weeks in the summer teaching in Munich, and it would be great to know the language.

I think about this question every day, as I've studied all of the languages you mention (which has opened some fascinating doors for comparative work). Before you launch into Chinese, you MUST read this fantastically funny and insightful article: Why Chinese Is So Damn Hard by David Moser (Michigan Center for Chinese Studies), http://pinyin.info/readings/texts/moser.html. I gave up on Chinese after trying it and reading this article, though I still find it fascinating and will continue to dabble. As for Arabic, the problem is its diversity, as seemingly every country has its own version (more different than British and American English). If you learn the standard "Fus'ha," you'll sound like Webster (or maybe even Shakespeare) to most natives, and it's a bear of a language, as well. Stick with Russian if you want a fun challenge that will provide lots of interesting comparative company law opportunities. Though it's dated now, I still recommend (and use with some private tutoring students) Ben Clark's textbook on beginning Russian.

As for me, I'm starting Polish now and continuing with Danish, but I must say that I agree with Kip and Orin that German has helped me more than any other language (at least in my law research). For fun, I would choose Greek (koine, not modern).

Your post seems to indicate that you are interested in learning a new language just for the fun of learning it, but it doesn't seem to take into account either the network effects of language (to whom are you going to speak this language) or the practical benefits (this language will help me to do [blank]). I would either pick Swedish (so you can talk to your wife in your own secret language) or the language your son picks (so you can learn it together and have your own secret language) or Spanish (so you can talk to the other half of the people on our continent). Otherwise, the exercise seems more like learning a code from WWII or Braille -- sort of a nice mental exercise.

Fun comments. Thanks.

Christine, you seem to place a very high value on secret languages! I guess I don't have that many secrets. This never occurred to me as a relevant consideration.

But I was trying to think practically. I can get around Europe pretty well with my German and base-level understanding of French. But Beijing was much harder, when I was not with my Chinese hosts. So I figured a little Mandarin might be nice.

Plus you're right about learning a language just for the fun of it. Languages are fun, and I think learning Mandarin would offer some interesting insights into China.

Jason ... wow. Obviously, you agree that learning languages can be fun, but Polish? There must be more to that story. Thanks for the tips, by the way.

Orin, I'm jealous. Munich is a wonderful city. I hope you take the time to do some side trips.

Kip, I thought about including Latin, but we were pretty focused on living languages. Speaking of which, Jeff ... keep riding that Esperanto horse!

Hi Gordon:

I'd pick Chinese or Arabic or perhaps Russian, for a different reason from those earlier stated: language is a really neat window on culture, and there may be more fascinating discoveries to be had with languages and cultures not so closely related to mainstream US culture.

For example, my understanding is that in Chinese, there was no generally accepted word for "democracy" until early in the 20th century. If you wanted to say "democracy," it took at least a few sentences. Conversely, there are Chinese words that have no one-word English translation, because a concept readily apprehensible in Chinese may need some explaining in English.

I grew up speaking Mandarin with my parents, but like many first-generation American-born kids, I stopped speaking it as soon as I hit grade school. I've relearned a bunch of it over my adult life, and so perhaps I have a special fondness for these sorts of cultural discoveries. No doubt, however, that most European languages (not Finnish) are probably easier to learn, though you won't get as big a secret language bang for your buck.

Anksthay, Ordongay.

Personally, I think a law professor should be able to say "conflate" and "normative" in any language.

Ouryay elcomeway, Effjay.

Is anyone aware of more immersion-like programs (that aren't either a million bucks or in the form of a plane ticket)? I learned English around the house, and it would be more valuable for me to just do it than it would to be able to explain verb conjugation academically.

I can't believe no one's voted for Spanish! It was the clear choice for me because of my confusing last name (Portuguese, by way of Goa, India). Everyone already presumed I spoke Spanish. But I never regretted the choice--I have found it so useful in this country, particularly in the south.

My next language is definitely German, although my French still needs work.

Sorry, guys, but after NonVoxPop, I have to explain why nobody knows English grammar:

Henry: Look at her, a prisoner of the gutter,
Condemned by every syllable she ever uttered.
By law she should be taken out and hung,
For the cold-blooded murder of the English tongue.

Eliza: Aaoooww!

Henry imitating: Aaoooww!

Heaven's! What a noise!
This is what the British population,
Calls an elementary education.

Pickering: Oh, Counsel, I think you picked a poor example.

Henry: Did I?
Hear them down in Soho square,
Dropping "h's" everywhere.
Speaking English anyway they like.
You sir, did you go to school?

Man: Wadaya tike me for, a fool?

Henry: No one taught him 'take' instead of 'tike!
Why can't the English teach their children how to speak?
This verbal class distinction, by now,
Should be antique. If you spoke as she does, sir,
Instead of the way you do,
Why, you might be selling flowers, too!
Hear a Yorkshireman, or worse,
Hear a Cornishman converse,
I'd rather hear a choir singing flat.
Chickens cackling in a barn Just like this one!

Eliza: Garn!

Henry: I ask you, sir, what sort of word is that?
It's "Aoooow" and "Garn" that keep her in her place.
Not her wretched clothes and dirty face.
Why can't the English teach their children how to speak?
This verbal class distinction by now should be antique.
If you spoke as she does, sir, Instead of the way you do,
Why, you might be selling flowers, too.
An Englishman's way of speaking absolutely classifies him,
The moment he talks he makes some other
Englishman despise him.
One common language I'm afraid we'll never get.
Oh, why can't the English learn to set
A good example to people whose
English is painful to your ears?
The Scotch and the Irish leave you close to tears.
There even are places where English completely
disappears. In America, they haven't used it for years!
Why can't the English teach their children how to speak?
Norwegians learn Norwegian; the Greeks have taught their
Greek. In France every Frenchman knows
his language fro "A" to "Zed"
The French never care what they do, actually,
as long as they pronounce in properly.
Arabians learn Arabian with the speed of summer lightning.
And Hebrews learn it backwards,
which is absolutely frightening.
But use proper English you're regarded as a freak.
Why can't the English,
Why can't the English learn to speak?

Arabic or Mandarin Chinese was exactly the choice I had boiled it down to when I was starting college in the 70s. I took Chinese and have never regretted it. The Arabists may disagree with me, and not having learned Arabic I confess that I'm not competent to make a fair comparison, but I think nothing can compare to the richness of the cultural treasury that's opened up to you when you learn Chinese. I have nothing against European languages, but in their approach to the world they are all pretty similar (subjects, verbs with conjugations, objects, etc.) Chinese is, needless to say, very different, and that's what makes it so interesting - as well as a lot harder to learn. But the premise of your question was that you would have enough time. I also read "Why Is Chinese So Damn Hard?" (referred to in a previous comment) and there's a lot of truth in it. The author recounts randomly picking up a Spanish newspaper after having studied three years of Chinese, and finding he could understand more of the newspaper (despite never having studied Spanish) than he could understand of a Chinese newspaper.

I's new der's a reson I'nt get into Har-verd.

I think Chinese will be the most important language for business after english in the next 10 years. I think even studying the language so you can understand the cultural differences in language like Jason says. Spanish might be more easily understood and learned and that is the best language to travel with for south america and europe.
http://www.cactuslanguage.com/en/home/en-us.php
Good luck, I am doing german to teach my son and speak with my girlfriends family, I enjoy that more than the french I did at school

Russian was pretty easy to pick up during a mission after learning German for 3 years before. If you know German well, you will pick up on the cases and declension easily. Plus, if you learn Russian well, it's not too hard to read Ukrainian, Polish and the rest of the Slavic languages.

The comments about Arabic are right. After 3 years, I had a decent understanding of fusha, but not much else.

Chinese has a steep learning curve, but once you get the hang of characters and tones, it's not to bad.

I studied Russian in the 1980s, after which the Soviet Union collapsed. I learned Spanish (girlfriend), and then Japanese (after which the Japanese economy collapsed). Then I studied Mandarin, and despite these efforts, the Chinese economy and/or government has yet to self-destruct.

I do a lot of international work these days, and of these languages, Spanish is by far the most useful. There's an old joke that a person who speaks 3 languages is trilingual, 2 languages bilingual, and a person who speaks one language is called an American. This is always said as a joke about American intelligence or our education system, but in reality it is a statement about the size of the United States. You can be in Chicago and go a thousand miles in any direction and everyone will still speak English (more or less). The same can be said for Latin America, just more so. English is the official language of business throughout the world and pretty much everywhere you will find someone who speaks English (even government agencies in China). The two most prominent exceptions are Japan (where the quality of English training is weak for some reason) and Latin America.

I don't think Chinese will replace English as a business language anytime soon. For one thing, business languages are always one "empire" behind (Greek was the universal language during the Roman Empire, Latin during the Middle Ages, French during the British Empire -- and now English during the American hegemony. If the Chinese replace the U.S. on the big pedestal, that still means English will remain the universal language. It's only when India or Brazil replaces China that Mandarin will start to catch on.) That said, Mandarin is very helpful. But for the most bang for your buck, Spanish is still king.