This is in no way meant to be a criticism, but does your child 'switch off' when he hears a language he doesn't understand? Many children do and many adults do as well. In a lot of situations, we don't actually need to understand the words, or certainly not all of them. We pick up clues from what is going on around us.

I had non Swiss German speaking children and one deaf child in a group I taught and to begin with they would say that they didn't understand me. I used to ask them what they thought I had probably said when every other child went and picked up a ball. This took the pressure off 'understanding each word' and made them aware of what was going on around them. Things improved by leaps and bounds then.

Hope you find a good solution.