I currently have the following rule:
period = {$season ->
[spring] Vår {NUMBER($year, useGrouping: false)}
*[autumn] Höst {NUMBER($year, useGrouping: false)}
}
As per the docs.
However, this just doesn't work, the rule does not apply and displays the untranslated fallback. It looks like the functions don't support boolean values, every other parameter that is a string or number works. If I pass useGrouping: 0 it works, which reinforces that.