This right here. I have been preaching this message for ages. You simply cannot account the immense benefits of having everyone working on the same stack. Even just splitting into two languages you start getting huge knowledge barriers and a major loss in shared skill growth. If you've chosen Java/C#/etc for your base language you need to have a DAMNED good reason for bringing in another one. And the other one being the best option for the problem of the day is not a good enough reason if your primary language is capable of solving the same problem in a slightly less elegant way.