Heck, I'm not even sure how things are supposed to work in some aspects. Let's say, with a locale setting of LC_CTYPE=de_DE.UTF-8, should all characters defined in Unicode match \w, or just the ones actually being used in the respective language/region?

IMHO there should be two primitives, for example \pL for matching Unicode letters, and [[:alpha:]] for locales-based letter matching.

\w could then die with "please be more specific in your choice of character class" if locales are in effect.