"if you don't understand French - you don't understand the French"
That's certainly true, though by no means unique to the French and their language. There is also the lack of shared cultural history, of course, which means that things that are obvious to French people are impenetrable to people who didn't grow up in that country. This is the reason that non-British people often do very badly on British quiz shows, especially with the initial "easy" questions - ask any Brit of a certain age which children's programme had the presenters John Noakes, Peter Purvis and Valerie Singleton and they'd have no trouble with the answer, but a French person would almost certainly have no idea what you were talking about!
Similarly, if a French person says "T'es complètement glécin, toi", are they insulting you, praising you or just having a bit of banter with you? Unless you'd grown up in France or can otherwise speak verlan, you'd be clueless.