I agree you should have a real problem to solve when learning a new language. However, I'd temper expectations about the outcome. Generally I find side project

I agree you should have a real problem to solve when learning a new language. However, I'd temper expectations about the outcome. Generally I find side projects are either about learning a new technology/language or making something that should exist, but not both. It's important to be clear which is the primary objective. If the objective is to make something, pick the most familiar language and libraries, bang it out, and ship it. If the objective is to learn something, for sure pick a real project to focus your efforts, but understand it will take longer and the end result probably won't be amazing, though hopefully you'll learn a lot in the process.