The question "What language to Learn Next?" and all the various derivatives of that same question has been answered countless times, both by the volunteers on this site, as well as the authors of numerous blogs, articles, career journals, and forums.
In fact, here is the same question What Should I Learn Next?. There are 8 answers there about what language to learn next.
This Programmers SE Search for "What Language Should I Learn" turns up 67 pages of results, with the first 2 pages dedicated to various different incarnations of the question
What Language Should I Learn Next?
Thus, if you want to know the answer to that question. Start there and work your way through the bounty of answers available. In addition to answers, you'll find links to other resources, which will link to other forums, documents, opinions, articles, and journals.
The goal is to not answer the same questions that have already been answered; instead, the volunteers here look forward to challenging new questions that have yet to be answered.