比萨は通常イタリアの地名として使われています。あの有名なガリレオ・ガリレイが「比萨斜塔」で有名な物理実験をやりました。英語は同じピサだけど、たぶん通訳は食べ物と地名の区別をつけるため、違った字を使っているんじゃないかと思われています。

Nothing to do with the dialect/geographical difference. Just different translations from the sound pizza. Since 披萨 sounds closest to pizza魔术贴people nowadays tend to use 披萨 or 匹萨 （same sound：pi1 sa） The leaning tower of Pisa in Italy is called 比萨斜塔 which is a fixed translation. I think calling pizza 比萨 comes from there. I feel like the old generation may still use 比萨 sometimes.

haha, maybe. In China, the translation between Taiwan, Hongkong and mainland is different. in addition, because of different dialects all over the country. I think you can call it different names according to different scenes. also call it as follow, 比萨（Pizza），又称为比萨饼、匹萨、批萨、披萨。 most of people will use 披萨 in my impression.

uhrrr the 魔术贴 in my last answer is completely a typo...