[Elia]

Nobody knows a language perfectly as there no exact numbers on how much words does it include and stuff. How much words did Alexander Pushkin know? Around 20 000 of actively used words, as his dictionary suggests... How much words does the average _educated_ russian use? Much less I believe...

Most people's understanding of how well a person knows any language is 'be able to use it in different contexts with different stylistic and metaphorical meaning'. Words are no language until you put them in perfect place.

Anyway, if you can joke in a language, you certainly have it in your head. Have it, don't try to learn it.