This one might be a bit controversial, but has rung true in my general experience. Probably a lot of exceptions to these rules, but here goes:

You don't really know a programming language until you understand a fair amount of the standard library and how packages/modules/dependencies work. Syntax is pretty easy, and any mainstream language will work just fine for solving basic leet-code style problems. But when you really spend a lot of time working with a language, you're going to spend more time learning about common libraries and how to manage dependencies. If you're working with a language like C++ or Java, this could also include build systems and how to use them.

Another precursor to being able to say that you know a language is that you should also be familiar with best practices (ie. how to name modules, how to write documentation, etc.) and common pitfalls (undefined behavior, etc.). This is one of the hardest parts about learning a new language in my opinion, because the language may not necessarily enforce these things, but doing them the wrong way can make your life very difficult.