Meta-answer: I like to pick a project I wouldn't mind doing a 2–3 times in that language. It's hard to learn all the best practices, idioms, etc. of a language

Meta-answer: I like to pick a project I wouldn't mind doing a 2–3 times in that language. It's hard to learn all the best practices, idioms, etc. of a language in one sweep. Having a project that you can repeatedly do in X language—making it more "X-like" each time—is a good way to learn how to refactor things and structure things "natively" in language X.