Recently, there was a forum conversation on the precise meaning of "Tu parles!" as an exclamation. This is something I heard quite often the last time I was in France, being around two adolescent boys (or rather, being around the mother of two adolescent boys), so I know what it means.
"Dude, you're gonna ace that test!" "Oh, tu parles! My science teacher gives the hardest tests, this is gonna be rough."
"Mom, I'm going outside!" "Tu parles! Not without your coat, you're not!"
I know how to use this phrase. So does almost everyone else who speaks French on that forum.
But then, as we started talking about it, we came up with about two dozen different translations for it. High register, low register, sarcastic, joking, ironic, disbelieving, scoffing, wishful... By the end, we all ended up more discouraged than anything else. I kept reading through everyone's responses again and again, thinking, do I really know how to speak French at all??
That's how translation works, though. You read a word that you know, you know what it means...just not in that particular context. You find a word that you could use everyday in any foreign conversation, but you have no idea how to express the same thought in English. Or you drag out your thesaurus to read through all the synonyms of "indolent," only to decide that the best word for the circumstance is "lazy." Nothing fancier.
Do really know how to speak English?
The debate rages on.