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