Your plan of attack and style of learning will affect your programming language choices. For instance, if you learn best in a face-to-face classroom setting, you need to make sure that classes exist for what you want to learn. For many nonmainstream languages, even the book choices are poor.
No matter what, you’ll want to make sure that you feel comfortable in the online communities for that language. Consider these questions: How do they treat newcomers? Do a lot of questions go unanswered? Is there a good mechanism for turning questions into better documentation?
On that note, is the documentation any good? A poorly documented programming language will force you to count on the community, and an unhelpful community can sink you pretty quickly