(I think your English is very good).
I wonder how much of your son's not understanding could have to do with the language barrier?
It is hard enough for children with autism to learn to communicate, and it sounds like your child may be doing it in two languages.
I know my own son went from hardly speaking at all to full sentences like BAM! It could be that when your child is comfortable enough with his speech, he will do the same.
If your child is socially having a good time in "regular" pre-school, then I think it is fine to leave him there, and may be seek spech therapy for him in addition.
If he is staying upset about school all the time though, then you might want to look at putting him in special ed.