At the very end of the movie, Inga is singing the "transylvanian lullaby", and Frederick suddenly changes, and makes that growl like the monster did earlier. Does that mean that he is now the one with the abnormal brain who can´t talk? Is he stupid for the rest of his life, like the monster before him?
It wasn't a one-way brain drain between Frederick and the Monster. The Monster can be more intelligent and civilized than he was before, and Frederick can be more straightforward and passionate. There's also a hint that Frederick somehow managed to develop his own version of The Monster's enormous schwanstucker.
But can Frederick still talk after the end, or just growl? Will he be like the monster was before?
About the schwanzstucker (btw, I´m german, "Schwanzstucker" is not a real word. The correct word would be "Schwanzstück", but nobody says that), I think that there is no hint about that - Inga is just screaming because he is so passionate, I think.
There's also a hint that Frederick somehow managed to develop his own version of The Monster's enormous schwanstucker.
At the appropriate moment, Inga bursts into the song 'Oh, sweet mystery of life, at last I've found you', as did Frederick's fiancé after the Monster ravaged her!
As for the word 'schwanzstucker'; the whole movie plays fast and loose with language and accents. Igor has a Cockney accent, Gene Hackman his normal American accent, though playing a hermit in Transylvania ... it's a Mel Brooks comedy. Just enjoy!
"You're so analytical! Sometimes you just have to let art... flow... over you." The Big Chill