Also, from one time when I was a temp at a primary school for 2 weeks. I saw a kid who had come to Norway at the age of 10 or so, and now one year later he could still not add double digit numbers. Worse yet, he didn't see how a problem could be solved by adding two numbers. So problems along the lines of "I have two apples, you give me three, how many do I have now?" where beyond his ability to solve without assistance.

He did however manage to learn quite good Norwegian in that year.