Jane Street makes use of OCaml's built-in metaprogramming facility, camlp4. While these are in some sense "our own extensions", they're wholly shareable without using a hacked version of the compiler, and just because we build our code using these syntax extensions, doesn't mean that users of our libraries need to do the same. You can use Core_kernel, for example, without using our syntax extensions.

The French thing is a non-issue. The compiler is written and documented in English, and all the main contributors are fluent English speakers, and the mailing lists are almost entirely in English.