When am I allowed to add a language to my list of languages that I’m familar with?

So I’ve been learning C# for a couple years, and what I’ve realized is that many functional programming languages are either rather similar to C# (if they are an OOP language) like Java or JavaScript, or they are much simpler (such as Python, Ruby, or Lua). So seeing how simple all these languages look after using C#, can I just blow through videos like this to learn the syntax and make sure there are no hidden surprises and then mark it down as known? I don’t plan on just “busting through” a difficult language like C, but most of these popular languages apply the same logic that I’ve already used and am familiar with.

EDIT: I’m not claiming to be able to master a language in a day, but merely grasp the basics in a day or two so it looks good on my resume when I say I know twenty different programming languages. So the question is, how much of a language must I learn so that I’m not lying when I say that I “know them”.