Adding Cultures after Go-Live

This is similar to an existing enhancement request PF-26868.
Unfortunately you can't add custom culture codes ( well, you can in the server config, but they won't actually appear in the application unless they are one of the standard culture codes as set my Microsoft )
If it is only a few variations of descriptions, then you could use some other "English" culture codes like en-AU, en-CA, en-CB, en-IE, en-NA, en-GB, en-ZA , and get your cube views to display the correct one depending on the entity.

You could of course do something more custom, using XFBR/BRString in the :Name override in the Cube View Rows/Columns, but then this won't work so well if you are using member expansions like A#BS.Tree, A#PL.Descendants.  A DynamicCalc on the V#Annotation (or other text-based view member) could also be used to show alternate descriptions based on some lookup logic, but will probably have a negative impact on performance.