When Not to Switch a Programming Language

This developer's conundrum: "Should I realy change programming languages at this point in the project?"

At one point in almost every developer's career, there is work to be done with an "inherited" project. What this means is you are handed the responsibility of maintaining, improving, and adding features to a project that had many, many decisions and collaborations cooked into it. 

I will be talking about one aspect of this, one which keeps me scratching my bald head: When should I choose to use a different programming language?