It is absolutely not the case that all problems worth solving are solved already. Programming language development isn't necessarily about being a genius but rather a willingness to put in a monumental amount of work. Writing a language that compiles is easy enough. Getting a language off the ground to an actually useful place is tedious, simply in terms of the sheer amount of work to be done. Specification, implementation, documentation, diagnostics, optimization, configuration, tooling support, and creating a standard library (especially a cross-platform one) are things that will mire you in many hundreds of hours of work.