plasma6-workspace is still built without support for dynamic locale generation.
I do not know what en_AT is supposed to be. If you intended to have English time/date - just copy (or link) /usr/lib/locale/en_US.utf8 to /usr/lib/locale/en_AT.utf8. If you really want to have some mix of English and (Austrian-)German - you would need to install glibc-i18ndata, edit locale definition and use localedef to generate it.