when is the moment you say that you know a language?

To me the average developer doesn't really know any language per se in the true sense of the word. I think you "know" when you know there's a tool, method or functionality that can be used to solve the problem you're attacking in that language. Then you go off to put your basic learned syntax with your dev experience + the research code you've found and accommodate it to your solution. When you're at that level, you "know". Imo.

When you can hack at the internals to fix bugs/add features.

Don't learn a language. Learn programming in its different flavors.

Then knowing the language means having a solid foundation for the syntax for not having to look up everything anymore. (You'll still will read lots of docs. But more on the API level of some framework / dependencies you use.

I recently watched a series in which the author rewrites some of the BSD shell commands as if it was 1994, in C. That's what I call knowing the language.

Never.

Maybe if I wrote one myself...

when you realize how big of a pile of shit it is

You never do

@BindView beat me to it.

If you're still thinking, "wow, I want to use this for loads of projects!" then you're still at stage 2.

Stage 1 is, "holy shit, I need to use this for EVERYTHING!".

For me it is with PHP. Although I of course don't know nearly everything about it, I hardly have to search for things I can't figure out anymore :)