I think you're right, and also it would be silly for a man like Jim not to know that there were other languages. The debate he and Huck have about French in Chapter Fourteen is hilarious and really compares racial prejudice to linguistic prejudice, but it's not founded on any kind of realistic assumption about Jim's knowledge of the world. I guess Twain is writing before the rules of realism have taken hold and he feels free to stretch the verisimilitude whenever he wants to get a good effect in the narrative. Hemingway said that the end of HF cheats, but Hem is playing according to different rules. Maybe this is another example of the rules of the game in the era of Twain, which might seem odd to us. Steve