What is an adjective relating to the concept of dialects?
dialectal; the third example is almost exactly the context you describe:
Dialectal differences between Canadian and American English were judged to be irrelevant for this research.
I will say that I personally would use "the differences in dialect" or "between the Castilian and Chilean dialect of Spanish", before "dialectal". "dialectal differences" sounds pretty technical, but that may be what you are going for.