I am just going on what Stallman says himself:
My favorite programming languages are Lisp and C. However, since around 1992 I have worked mainly on free software activism, which means I am too busy to do much programming. Around 2008 I stopped doing programming projects. As a result, I have not had time or occasion to learn newer languages such as Perl, Python, PHP, Ruby, Lua, Go, Scala, Rust, and so on. Therefore, I don't have an opinion about them as languages.
If I were not so overloaded with responsibilities and work that is important to me, I would study them out of curiosity. But that is not likely to ever happen.
If you want to learn C, but you already know how to program in some other language, I recommend you try my GNU C Language Introduction and Reference Manual.
I read a book about Java, and found it an elegant further development from C. But I have never used it.