Gaulish language The answer to your question in Gaulish language is that the socalled Vulgar Latin which should be called a Latinized ItaloCeltic idiom instead because the spoken language of the Romans OscoUmbrian or Sabellic language and not Latin already essentially a written language the ProtoItalian was so similar to the Continental Celtic languages that only the lexicon and the pronunciation differed a bit and could almost be considered a dialect of Gaulish as it contained the same substratum unlike the Insular Celtic languages   So in reality no one can assert that Gaulish is an extinct language because we dont know enough about it before the Roman conquest and probably many Latin words were already used by the Gauls before their first contact with the conquerers At the most assimilation could better describe what happened to that language Both the ProtoItalics and the ProtoCelts Veneti originally come from the BellBeaker culture and lived in a dialect continuum for a long time In fact Celtic is more a culture brought by a ThracoCimmerian aristocracy than a language family except for Brythonic and Gaelic languages which are both quite different from each other because of their respective substratum as mentionned above Moreover ProtoEnglish or ProtoFrisian spoken by the Belgae was already used in Britain as well as in some parts of northern Gaul Gallia Belgica by the time of the Roman invasion then the Belgic tribes just kept speaking the same language during and after the Roman time