Perhaps implicit in the advice to use a single language is the assumption that the set of things you will have to write with it is bounded. If you are a web de

Perhaps implicit in the advice to use a single language is the assumption that the set of things you will have to write with it is bounded. If you are a web development shop, you will not be writing Tensorflow or device drivers. You don't need to worry about picking a language which can do those. You are quite write that you shouldn't write a web frontend in Java, though. For web development, in practice, anyone picking a language which is not JS/TS will have to use two languages. That's still better than more than two!