Thomas Jefferson himself said in his Notes of a Tour through Holland and the Rhine Valley, "...there was not a person to be found in Duysberg who could understand either English, French, Italian or Latin. So I could make no enquiry."
And later in his Hints to Americans Travelling in Europe, "I could find no body in the village however who could speak any language I spoke, and could not make them understand what I wished to see."
Jefferson said in an April 12, 1817 letter to Joseph Delaplaine, "I was educated at William and Mary college in Williamsburg. I read Greek, Latin, French, Italian, Spanish, and English of course, with something of it's radix the Anglo-Saxon." In addition to the languages he lists, there is some evidence that Jefferson was attempting German.
Jefferson had dictionaries, vocabularies, and grammars in a number of other languages in his library. These included Arabic, Gaelic, and Welsh, amongst others. However, without confirmation from Jefferson himself, the most we can assume is that he was "dabbling" in these languages, and never achieved a notable degree of fluency.