I'm often asked why Tarantool is written in Objective C. Damien Katz, my ex-colleague from MySQL AB :), wrote a very good post on the strengths of C. We use Objective C as "C with exceptions". Objective C' @finally clause allows for simple integration of exception-aware code with C code. In contrast, the only sensible way to deal with exceptions in C++ is RAII, and this pretty much means that you forget about C the moment you decide to use exceptions in your program.
One serious "deficiency" of C is that it doesn't bring along the programming paradigms and patterns found in modern programming languages. In other words, it doesn't teach you programming culture. This is why, I think, it is
much better to return to C after a few years with other languages. There is something unique to learn in almost every modern programming language.
Often, a larger project uses a ton of languages and instruments. Tarantool is not an exception: apart from C and Objective C we use Lua, Ragel, Bison, a configuration file parser of our own breed, and this is just for the server itself. For tests, we use Python, Perl, PHP and Shell. Some of our benchmarks are written in Java.