Perusing the Anthropologist in the Attic, I happened on 37 Languages, written by a fellow Louisvillian. This blogger is reviewing a substantial number of languages as part of a process to settle on one that he would like to learn fluently.
I don't know if it's really a great way of going about this, but the project has captured my attention for some reason. His approach is relatively systematic, examining each language one by one, and relatively intensive, doing some of the work of studying the language seriously (such as learning to read and write its script). But his criteria is also idiosyncratic and aesthetic, and that subjective aesthetic assessment of a variety of languages is interesting. Maybe it's merely that, instead of learning some ~about~ the languages he might want to speak, he's learning some ~of~ those languages. If a smattering only.
Gaining a passing familiarity with a foreign language is fairly easy, but gaining fluency is fairly difficult: you have to master a different sound system, internalize a number of rules, develop strong fluency and listening comprehension, and memorize some tens of thousands of lexical items. It's a lot of work, usually for only occasional or intangible benefits; the ten-thousand-hour hypothesis may be relevant. Developing real fluency takes sustained learning over a period of years, during which time interests will shift and motivation will wax and wane. A language you choose to learn should be one that'll you'll be able to maintain a high level of motivation and interest in for a long time. The process of choosing a language is important, and it's probably an area that prospective language students don't spend enough effort on.
In terms of developing future fluency in natural languages, I'm probably limited by a certain path dependence: I've attained a certain proficiency in Japanese, which I would like to maintain and extend, which is fairly laborious. And the practical merits of Spanish will likely make it my next main target. But examining languages in the way that the 37 Languages blog has seems like taking "language vacations" away from my main interests, which sounds like fun.
So. If I were to do something like this, what languages would I nominate? Probably something like: Old English, Latin, Coptic, Greek, Hindi, Farsi, Icelandic, Frisian, Armenian, Spanish, Welsh, Irish, Gaelic, Cornish, Czech, Russian, Evenki, Ainu, Tibetan, Korean, Hungarian, Turkish, Tatar, Tuvan, Mongol, Arabic, Kriol, Fon, Khoekhoe, Iñupiaq, Hawaiian, Myaamia, Chickasaw, a Dhegiha Siouan language, Cherokee, Caddo, Nahuatl, and Quechua.
That's 38. Lots of those languages sound as if they'd be great to visit, even if I couldn't live there. The list includes some widely varied languages, and a few clusters of closely related languages, which seems like a good way to pick between competing options. The main things to overview would be some background; obvious hurdles; phonology and orthography; cultural, linguistic, and aesthetic appeal; and educational resources.
The Internet has heralded such a golden age of language learning opportunities; it's like the opposite of the Tower of Babel. And helps information about language revitalization spreading over the globe.
A nice discussion of "Difficult languages" at Language Log.