The trouble with assessing children in their native language is that though this may be what they heard and spoke from home, they have NOT been schooled in their native language – and standardised assessments ( even the wisc) have norms that ASSUME the input of that language in an educational setting. So the data collected cannot be used in the same way as a native speaker living in his native country and attending his native school. Especially if the home is a home where parents have not been exposed to education.