Can you give an example of one of your exam questions? I'd like to understand how English comprehension poses a barrier to a physic problem. Perhaps your exam questions are reworded in such a way to prevent a student from memorizing solutions or lifting their solution directly from an example in the book? If the problem statement is something never seen before then take care no to make it too difficult for non-native speakers to comprehend. If you do then you risk giving non-native speakers the disadvantage if they cannot use dictionaries.