If you can wrap your head around advanced C++ concepts and idioms, learning other languages will be a walk in the park. All the knowledge will be 100% transferable to anything you use in future. Also a lot of companies using java, c# etc will take c++ experience as relevant experience even though you aren't a java/c# expert, but the inverse is not true.
It's never a bad idea to learn something that has shaped essentially everything that has come along since.
It's never a bad idea to learn something that has shaped essentially everything that has come along since.