There is this paragraph in ABOUT-GCC_NLS:

By and large, only diagnostic messages have been internationalized.
Some work remains in other areas; for example, GCC does not yet allow
non-ASCII letters in identifiers.

It seems like this hasn't been true since GCC 10. I can see in release 
notes:

https://gcc.gnu.org/gcc-10/changes.html

Extended characters in identifiers may now be specified directly in the 
input encoding (UTF-8, by default), in addition to the UCN syntax 
(\uNNNN or \UNNNNNNNN) that is already supported:

static const int π = 3;
int get_naïve_pi() {
   return π;
}

I think this either needs to be updated or removed completely?