This happens for Brunei, where the week-end is Friday + Sunday
Our data structures don't allow us to represent that info.
CLDR data: the `firstDay` elements in `supplementalData.xml`
And in Java we have Calendar.WeekData
In C++ there is not such class, but in C++ we have `icu::Calendar::getDayOfWeekType` and `icu::Calendar::isWeekend`
And and Java we have `Calendar.isWeekend` and `Calendar.getDayOfWeekType` (deprecated)