[qwytw]

IIRC they were using Python initially. Turned out it was way too slow (also it was the mid 2000s).

They hired the guy who created Boo (a python like programming language) and switched to Mono (I'm not sure why they picked it instead of JVM I guess it was possible related to licensing but because they just had people who were experienced with it).

Also C# wasn't even their primarily programming language until 2012-2014. It was UnityScript (JS like syntax running on Mono). The idea was that C#/static typing was too scary/complex for most of their uses.