That would be very confusing for multilingual users. Just because my OS is configured to use a certain locale, doesn't mean I don't read text in scripts not considered part of it.
They are of course, but if you use that list instead of a single locale, you end up with a solution that only highlights 'strange' characters when they are not part of your language/locale set. So for someone who speaks only Latin character based languages you could highlight all Cyrillic characters, but for someone who speaks Russian you still have the original problem (it's not as if you can just highlight all Latin characters in their case!).