Your Turkish example doesn't work with `fd` because `fd` doesn't support specific locales or locale specific tailoring for case insensitive matching. It only supports what Unicode calls "simple case folding." It works for things far beyond English, as demonstrated above, but definitely misses some cases specific to particular locales.
TR18 used to have a Level 3[1] with the kind of locale-specific custom tailoring support found in GNU's implementation of POSIX locales, but it was so fraught that it was retracted completely some years ago.
That's false. Counter-example:
Your Turkish example doesn't work with `fd` because `fd` doesn't support specific locales or locale specific tailoring for case insensitive matching. It only supports what Unicode calls "simple case folding." It works for things far beyond English, as demonstrated above, but definitely misses some cases specific to particular locales.