My take on this:
1) you don't need to understand every single word in a sentence to understand what is being said. That's basically the first thing that you have to accept when you learn a new language. Even in my mother tongue there are plenty of words I don't know.
2) Urianger's way of speaking is mostly a joke at this point. It's just as funny as Mikoto using her scientific lingo that nobody understands in the Ivalice/Bozja questline.