Viaggero: since you don't seem to have received an explicit answer to your question yet, I'll volunteer this: I took a series of courses in Quechua, the major surviving indigenous language of the Andes, a few years ago, and though I can still hardly claim to be fluent, I enjoyed the study very much. My reasons were: an interest in languages in general (the essential reason); a desire to take on a language unrelated to English; a general interest in Latin America; and, shortly before, my first visit to Peru. It was interesting to see how this language worked, and especially to discover how logical and regular it is. (I realize that "logical" is a tricky adjective to use with reagrd to languages, but in the case of Quechua I think it can be defended. I know the French think their language is "logical," though I've never understood why -- and anyway, like most other European languages, it's quite a mess compared to Quechua.) It's too bad the Spanish cut the Inca empire short so early in its career -- had it continued to flourish, perhaps Quechua would have become an international language, and we would all have been better off for it!
(Incidentally, the estimated number of first-language Quechua speakers varies between 6 and 8 million, but I suppose by most standards you could still regard it as a "lesser-used" language.)
