Complex sentence: Dialects of Italian are regional varieties, more commonly and more accurately referred to as Regional Italian, with features of all sorts, most notably phonological and lexical, percolating from the underlying substrate languages.
Split into simpler sentences: Dialects of Italian are regional varieties of the Italian language, more commonly and more accurately referred to as Regional Italian. The dialects have features, most notably phonological and lexical, percolating from the underlying substrate languages.