I'm happy to report that I managed to summon some... energy to study some more Welsh, and figured out a pretty complex sentence without requiring anything more than a short reference article and Wikipedia... Maybe you'll also find it interesting that the sentence consists of: negation before the main verb, then the subject, then the verb-noun "to have" + a possessive pronoun and another verb, all that used to convey the passive voice... Well, I suppose that's more confusing than interesting. Maybe a garbled illustration will help? Literally translated, the sentence goes "not is the term 'principality' to have its employ", meaning "the term 'principality' is not employed". Fun, right?

Oh, yeah, I also referred to Google Translate. :P