* German: His native language, obviously!
* French: He spent time in France as a young man, and his mastery of the language was said to be excellent. This was crucial in his diplomatic dealings with France.
* English: He had a good understanding of English, and was able to converse in it.
* Latin: As was common in the era, he had a strong command of Latin, which was the language of scholarship and diplomacy at the time.
It's possible he had some knowledge of other languages as well, but these are the ones definitively known.
While his fluency in German, French and Latin were particularly important for his political career, his understanding of English likely aided his diplomatic endeavors with Great Britain.