In C (POSIX.1-2008, specifically), there's tolower_l() and the rest of the _l functions for this use case, which take a locale as an argument. That let's you ask for the English (or even "C locale") lowercase versions of these English words, even when your process's current locale is Turkish.
https://www.man7.org/linux/man-pages/man3/tolower_l.3.html