Mainframes use bytecode based executables since ages, you AOT to native code at installation time, when there are critical updates or hardware changes.
Only the kernel and drivers are straight native code, this is what keeps stuff from the old days running on modern mainframes, without any change to the executables, although their hardware is completely different from the 70's models.
What is a "language environment" in this context? What would be some of the mainframe language environments that support C and C++?