Complex sentence: The Breton language is Brittonic, not Gaulish, though there may be some input from the latter, having been introduced from Southwestern regions of Britain in the post-Roman era and having evolved into Breton--still partially intelligible by modern Welsh and Cornish speakers.
Split into simpler sentences: The Breton language is Brittonic, not Gaulish, though there may be some input from the latter. They brought with them their Brittonic language, which evolved into Breton--still partially intelligible by modern Welsh and Cornish speakers.