Which language should you learn? - A guide to help you if you have no clue
I see a similar question every week on average, if not more often. I figured this article can help you decide if you have absolutely no clue which language you should learn next.
Personally, I'd say subjective, personal reasons should be the most relevant factor. If you have in-laws or extended family who speak it, or a particular interest in the culture of a country where it is spoken, for example.
The article gives a very good summary of the main factors to consider.