I'd say I know a language when I have an understanding of the syntax, I know the important fundamentals of the language, like its type system etc. and that I have built anything in it and I wasn't just following along with someone else.
I also might lower this barrier if I'm familiar enough with languages like it and have a good idea of what getting going with it might be like. I'm pretty sure I've never written a line of Python in my life, but I feel like I know the language from coming across it so often and it being pretty straightforward.
If I feel like I could take on a task with the language without a really long ramp up time, I'd say I know the language.
If you asked me to write something in Haskell, I would have no idea where to start, and could not guarantee I'd be up and running any time soon. Even though I know what the languages is about, I can confidently say I don't know the language.
I think the thing that throws people is the difference between the language and the frameworks and standard patterns and practices that are used in the language. I would expect most people can learn the syntax of a new language in a few days (obviously that might change depending on the language).
My personal "main" language is C#. I expect anyone with a background in C-style languages could probably figure out any normal C# code with just a little bit of time. Figuring out the innards of how to use C# to make a WPF app or an ASP.NET Core Web application suddenly requires significantly more research and time. I'm using .NET examples here, but you can replace C# with Java and WPF and ASP.NET with Swing and Tomcat pretty easily.
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