Knowing a language is helping you read/write the code in a good quality.
Knowing the ecosystem and solutions in this language is your sellingpoint.
Take any language and tell me one can be productive in no time without knowing the ecosystem around it.
Within our team we use 5 different mainstream languages to accomplish different goals within our system.
We handle a single service, but we manage the full stack using many different languages and frameworks.
We don't have to be expert in the frameworks, we just need to be experts at what we are building. And we learn each framework to the point where we can accomplish that.
The "selling point" that we value is to be able to deliver.
Knowing a language is helping you read/write the code in a good quality. Knowing the ecosystem and solutions in this language is your sellingpoint. Take any language and tell me one can be productive in no time without knowing the ecosystem around it.