It increases complexity. Your build system needs to be configured to support all the languages, you need code style guidelines for each of the languages, you ne

It increases complexity. Your build system needs to be configured to support all the languages, you need code style guidelines for each of the languages, you need rules about when to use each language, you need to ramp up each new team member on the languages, etc. All this is overcomable of course, but it stacks the odds against using multiple languages.