Most good devs - and basically all devs who make it to L5+ (Senior SWE) - will end up working in multiple languages, though. There are a bunch of proprietary ones as well, some of which are open-source (like Protobufs and Bazel) and a few I'm not at liberty to discuss. L6+ your soft skills (communication, EQ, management, prioritization) become more important than your technical skills.