2.9BSD, being a self-hosting operating system, includes all the source code and all the tools necessary to build it. That includes the C compiler, the assembler, and everything. There is no mention anywhere of any of this being renamed for DEMOS, so it's a safe bet that they had the same name.
DEMOS is similar in spirit to something like Russian Fedora Remix, which is a Linux distribution (everything is named the same, it is GNU/Linux after all), but ships with conveniences like a Russian keyboard layout and translations of user-friendly error messages. But you'd expect
make to still invoke
The Latin letter C is unrelated to the Russian letter С, even though they look the same and have (sometimes) the same pronunciation. The latter is descended from the Greek sigma, like our S is. C, the programming language, was named for being the next after B, so Russian names like С or К wouldn't really make much sense as a name for C. The language is known as C in Russia, (sometimes that's transliterated as "Си" to spell out the sound, "see"). But the compilers are called "GCC", "clang", "cc" or whatever. And as explained in your other question, the filenames, i.e. the commands are the same on DEMOS as on 2.9BSD.