Intl.DateTimeFormat: Why is timeZoneName: "shortOffset" is having a different value in different languages

A few things:

• Localized text of Intl API responses originates from Unicode CLDR (not IANA).
• The specific field in the CLDR data that controls the output you're seeing is called gmtFormat internally. For example, you can see here in French it's specified as UTC{0} where here in English it's specified as GMT{0}.
• As for why it's not consistent, I searched and found only one issue discussing the matter, CLDR-14121 which has been open under investigation for several years now. I left a comment there linking back here.
• You could always do some string replacing on the results to match your expectations, but keep in mind that CLDR data is updated periodically, so you may find things have changed over time without your intervention. Also keep in mind not every language has strictly "UTC" or "GMT" in this field (see GMT Format in this chart), or uses it at all (Chinese, Japanese, etc.), so replacements might not always fit every language.