Symptoms
at_AT.po locale file cannot be added to PCP > System > Settings > Locales. It does not appear in the locales list.
Same time, de_AT.po locale file works correctly.
Cause
Incorrect language specification usage: there is no language code "at" in the "ISO 639-1" standard.
Resolution
Create locale file with correct language code name.
Please refer to the INTERNATIONALIZATION AND LOCALIZATION article on apsstandard.org for more details.