(Former employee of quite a few years). The C++ usage is high due to a lot of legacy systems written in C++, however most new development (unless performance critical is not written in C++. I would estimate that Bloomberg actually has more JavaScript than it has C++.

When referring to the Terminal itself it's important to note that as a 'system' itself, it's incredibly broad and complex. You can do so many things via the system, it's equivalent to the broadness of a web-browser. In fact the Terminal application is powered by chromium these days. But the systems it connects to can be anything from trading systems, to Chat applications.