Complex sentence: Languages branch into dialects and thence into related languages through small changes in the course of children's learning processes which accumulate over generations, and when speech communities do not communicate (frequently) with each other, these cumulative changes diverge.
Split into simpler sentences: Languages branch into dialects and thence into related languages through small changes in the course of children's learning processes which accumulate over generations. When speech communities do not communicate (frequently) with each other, these cumulative changes diverge.