The kernel (WolframEngine) was built with c, and some parts like the image processing functions are built using c++. The front end (notebook interface) is c++ and some objective-c. A lot of core functionality is built using wolfram language that is loaded as needed. Similarly, the paclets (plug-ins) can be built using any language that has a WSTP interface available, but largely this is wolfram language and c++ and occasionally java, but that is being phased out. A lot of the functionality now lives in the paclets as they can be updated/fixed easily.

Wikipedia says Java: https://en.wikipedia.org/wiki/Wolfram_Mathematica

Mathematica predates Java by several years. Some capabilities (e.g. database interop) were built with Java though.