I am of the opposite opinion these days.

I know Python well. I know Django well. Those cover 90% of my needs as a programmer and the libraries are usually of a decent quality (compared to my experiences in JavaScript for example) so why would I introduce a new language? I will write better quality Python/ Django code because I know the idioms, and best practices, I know how to write clean code. If I switch to another language, sure I will be able to write the same, but the quality of my code will not be as high and it will take me longer.

I don't think knowing more languages is a bad thing, but adding new languages to a project for the sake of it can be.