You're right, the kernel and OS stuff are written in C. Take Ubuntu for example, most apps in C or C++ (med-low level), the user Interface is a mix of C++, C (same) and python usually with some C# for notifications, installer, and a user control panel (high level). it depends on the importance of it's running speed vs. the importance of it's development speed.

Notice how it's mostly C or C++, low level fast or mid level decent plus good development time? They don't use many obscure languages either, you'll find that among successful endeavors.

But mainly you have to pick, you can't be worried about speed over development time (it is low level and fast!) and development over speed "it's OOP and has high level features!". Also having something with more features translate to C does not give it the same speed as if you wrote it in C, it just gives you the same level unoptimized low level code as just writing it in a high level language the end.