As a CS student who's still not very confident in his skills and without enough experience to recognize his own weak spots, I would say that "you understand X's language well, but you need to improve your knowledge of algorithms and data structures" or "you have a good theoretical understanding X and Y Computer Science concepts but you need to work on side projects to gain programming experience" or "you have shallow understanding of a lot of languages/technologies but I advice you to concentrate on X or Y until you become good at them instead of jumping between L/T".

I know that very few employers would take the risk to be so frank but I personally wish they were. It would be helpful to the candidates who actually care about improving their knowledge and skills while potentially offending those who are trying to fake it.