A Tale of Two Libcs (opens in new tab)

In the "C" locale, it is required only to support ASCII. "In the 'C' locale, isalpha returns true only for the characters for which isupper or islower is true." N2176 7.4.1.1. "In the 'C' locale, islower returns true only for the lowercase letters (as defined in 5.2.1)." 7.4.1.7. "In the 'C' locale, isupper returns true only for the uppercase letters (as defined in 5.2.1)." 7.4.1.11. Section 5.2.1 in turn only defines "the 26 [basic] uppercase letters of the Latin alphabet [and] the 26 [basic] lowercase letters of the Latin alphabet." 5.2.1(3).

Even when a specific locale is set, it can only support ANSI, since the value must be representable as an unsigned char.

To do anything like the "right answer" in a Unicode world, you need to cast to wide int (wint_t) and call iswalpha, which is likely to be almost as broken. A thing that happens is that C libraries will rely on OS locale character set definitions, which in turn won't be installed except for those languages that are installed. You don't want Armenian Linux? Well guess you can't tell what's punctuation vs what's a letter anymore.

locale is a mistake and should have been depreciated 25 years ago.

The glibc version also only supports ASCII but instead of returning 'false' for out of range inputs, crashes your program.