There used to be requests for the “Euro English” locale in the past.
If the interest is still there, it could accumulate a critical mass to get such locale added for all users who’d want the European metric system (which navigation and measurement apps could default to), ISO date format, week starting on Monday, time shown as 24-hours, and I’m sure the €uro symbol would be prevalent in any currency-related apps in the Jolla store (and definitely in the Android world, where AppSupport defaults to Sailfish OS locale).
However all such users would still prefer Sailfish OS in English, and this is why such posts used to come up: https://together.jolla.com/question/134467/official-announcement-translate-sailfish-os-elections-of-community-languages-linguists/?answer=167773#post-id-167773 (Stack Exchange link within is also of interest, where en_DK and en_IE are being considered.)