I agree, all these preferences (currency, time, date, measurements) shouldn't need to fit in a determined locale box, but if you customize them it's possible to run into issues. I believe choosing English (Ireland) might solve your issue? Either that or I have found a way to customize it way back and then forgot.
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Plasma 6 Bugs
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Thanks! I'll keep English Ireland to avoid further mess.
For the calendar plasmoid you can set the preference for the first day of the week independently of your locale, in case that's all you need.
Thanks! That option also works, at least for the clock widget! And I'll be also choosing English - Ireland for now, hopefully this one sorts the rest of the formatting issues.