I have the advantage of still being in school, so all of this dry stuff is yet burning brightly in my brain. Adjectives derived from proper nouns are proper adjectives. Thus, whatever the nationality, the adjective would still be capitalized for an English tart, an American tart, or a German tart. Just one question, though. What makes you think she is a tart?
Go you! It was self depreciating i.e tart. Anyway I like tarts! Most of the annoyance with the spellchecker stems from being corrected by a machine, I'm one of those arty types that finds myself roaring #@%& OFF!! at the computer quite a lot. Apparently the guy that lives in the flat underneath mine once heard me bollocking my computer when there was another guy walking past his window; the passer by was not amused despite my neighbour protesting his innocence.