[discreteevent]

Of course they can learn new languages. But it takes some time to become really proficient in a language. The question is how many languages do you want to support in your company? Because it's not realistic to think that developers will not need to move from project to project. And in fact when things get busy they may need to swap from one project to another and back on the same day. It's not ideal but that's life. The inefficiency of swapping between languages will often outweigh the efficiency of the new language.

Google were notoriously strict for limiting languages at the start and I think it was very frustrating for some of the developers they hired. If I had a boring technology checklist then "Language we already use" would be top of the list.