Translation:You made a mistake translating that word.
There isn't an "in" before "translating" because that would change the meaning.
"You made a mistake translating that word" means exactly what the French says: you mistranslated that word.
"You made a mistake in translating that word" means that your error was the mere act of translating. It's not a likely thing to say in the context of translation, and the distinction is easier to understand if you think of a similar sentence: "You made a mistake in arguing with me."