Bonjour Alison, I think the better translation would be: You always mispronounced this word, so the French phrase would be: Tu prononçais toujours mal ce mot. The French equivalent of mispronounce, from the Collins-Robert Dictionary is «mal prononcer» or syntactically «prononcer mal» J'espère que cela vous aidera et bonne chance.

I think agruing that it's wrong to say "I pronounced it wrong" is imposing excessive, artificial logic on English. The argument is that a verb requires an adverb; you can't apply an adjective to a verb. But we say "I made it small," for example--we don't require one to say "I made it smally."