Celtic texts are found throughout Switzerland and the Po Valley as shown by the markers on this map. Most are short inscriptions on pottery with the Bronze Liver from Piacenza being a notable exception. Because of its long inscription the Piacenza liver was incorrectly classified as Etruscan.
Celtic culture is a more general term for the Hallstatt and Le Tene cultures who acquired their wealth from salt out of Salzburg, Austria and the trade along the Danube/Rhine axis. People from Malta at least played a part in this culture's origin.
Map from website: The Cisalpine Celtic Languages which is an ongoing European research project edited by David Stifter, Martin Braun, Corinna Salomon, Michela Vignoli et al. It is online at: https://lexlep.univie.ac.at/wiki/The_Cisalpine_Celtic_Languages#Definition_of_%22Ligurian%22