I submitted a service request and got an answer, so I'll put it here in case anyone else needs it:-
I will make an Enhancement Request to make it possible in the future to change the
Primary Language for a packaged application.
I will let you know the number of this enhancement later.
A workaround could be the following for now (I tried this and it worked):
1. Login to your database with user sys.
2. Perform the following query:
select workspace, to_char(application_group_id), application_id, APPLICATION_PRIMARY_LANGUAGE from apex_040200.apex_applications where workspace not in ('INTERNAL', 'COM.ORACLE.APEX.REPOSITORY') and application_name='Data Reporter';
This will return all Applications 'Data Reporter' in your APEX environment.
Find out the application_id in the row that is the one for your workspace.
3. Now we will update the primary language for this application with the following statement:
update apex_040200.wwv_flows set flow_language='en-gb' where id=<application_id you found at the previous step> and name='Data Reporter';
Interesting issue and suggested enhancement request.
I actually see this as being 2 enhancement requests. It most certainly would be useful to change the default language settings for the application as a whole (which you have demonstrated already, granted.....through unsupported and dangerous means), but I believe there should also be a user preference - because I could easily see different users of the same system needing access to the same reports in Data Reporter, and having different globalization settings.
Thanks again for pointing out this issue and making this enhancement request.