[tsmarsh]

What makes you think it's even possible to master a single language without learning others?

My experience tells me that monoglots reach false maxima because there experience isn't broad enough to realize that there are other maxima to climb.

Java is still, probably, my strongest language, but I strongly believe my craft would have stagnated around Spring/Hibernate if I hadn't taken the time to figure out how problems like typing, dependency injection, persistence, IO etc all work in other ecosystems.

Pick a language that is good for the market place you care most about. Mobile go with C or Swift. Science probably python. JavaScript if you like the web and if you like the perks of working in an Enterprise Java won't hurt.