"I don't speak potty mouth", to me, means "I'm a fucking prissy hoitey toitey goody two shoes and won't respond to gutter language" not "I don't know what that phrase means".
In the case of DB, it means:
"I don't understand anything that was written but I recognise a swear word, so I'll pretend to take the moral high ground. Not addressing the point raised removes the need for comprehension. That means I can avoid people noticing that I can barely understand written english."