I don't think this is true anymore. From the TIOBE top 10 (that hallowed source of truth), I count only three languages whose main implementation is written in C (Python, PHP, R). Extending to top 20 only adds two more (Perl and Ruby). All of those are interpreters. Most compilers seem to be implemented in the language they are compiling. It appears that C++ is the most popular implementation language, although C is number two - but neither constitute a majority.
