What language are system libraries written in?

Question

Windows

Tell me, are the Win32 system libraries (for example, kernel.dll, ole32.dll, mso.dll) written in a particular language or their family, or not?
I'm interested in the possibility of decompiling some of them. Who has experience, please share in a nutshell - success and the program that they used (for example, IDA pro 6.6 HexRays or so on.).

Answer 1

They are written in C, possibly with assembler inserts. The source code for their functional counterparts can be found in reactOS or wine.

Answer 2

Decompiling them to source will not work, but IDA can load PDBs, which can greatly simplify analysis.