There is no default language as far as I can see, rather the language is guessed in case no hint or an unknown hint is provided, this heuristic depends very much on the enabled language and the implementation in the highlighting plugin. In most cases the snippets are correctly identified as Fortran, but not always.
I see. As it has always worked (recognizing my Fortran snippets without a hint), I thought it was the default, which would not be a bad idea on a Fortran Discourse, anyway.
BTW, what is the proper hint for C snippets? The following gets properly prettyfied with just raw ``` but fails to do it either with c or C hint. Strange.
#include <stdio.h>
int i=1;
printf("%d\n",i);
while (i<5) { printf("%d\n",++i); }