Why not just write the dialog like that?
- "qcfhd skfnuk jenf b elfk?"
- "What?"
- "Do skfnuk jenf to elfk?"'
- "I'm not really understanding you."
- ""Do you jenf to elfk?"
- "Oh... do I what?"
Just use gibberish to what he doesn't understand, and replace the words as he learns them. You can even throw some "miss understood" words in the mix to add to the confusion. That can give you some funny dialogs (and eventually frustrating for the character) while he tries to speak using or understanding a wrong word here and there...
Ps - I would not use any recognizable language for the gibberish, since there's always the risk that a player will know that language, and then you lose the effect.
- "qcfhd skfnuk jenf b elfk?"
- "What?"
- "Do skfnuk jenf to elfk?"'
- "I'm not really understanding you."
- ""Do you jenf to elfk?"
- "Oh... do I what?"
Just use gibberish to what he doesn't understand, and replace the words as he learns them. You can even throw some "miss understood" words in the mix to add to the confusion. That can give you some funny dialogs (and eventually frustrating for the character) while he tries to speak using or understanding a wrong word here and there...

Ps - I would not use any recognizable language for the gibberish, since there's always the risk that a player will know that language, and then you lose the effect.