Every programmer should know many language. Dave Thomas and Andy Hunt once said (in "The Pragmatic Programmer): Learn a new language every year. This is good advice.
Every programmer should know a C based language like C, Go, Java, C# or C++.
Every programmer should know a functional language like Clojure, or F#.
Every programmer should know a stack based language like Forth.
Every programmer should know a logic language like prolog.
Every programmer should know LISP.
And that's just for starters.