They can. But when a person utters a sentence, they generally intend to state the derelativized proposition indicated by the sentence in their language. When I say "P", I don't mean ""P" is a true sentence in all languages at all places", I mean P(current context).

Which is why it's useless to say "I have a different definition of 'should'", because the original speaker wasn't talking about definitions, they were talking about whatever it is "should" actually refers to in their actual language.

(I actually thought of mentioning that the sky isn't always blue in all situations, but decided not to.)