Currently Postorius handles form submissions like this:
Currently the HttpErrors are passed without modification to the user in a notification. Postorius should have some way of translating these. We can't translate the raw messages because they sometimes contain specific strings like emails and it would require us to manually add the strings to the .pot files.
Theoretically the core could translate the messages before passing them to postorius. This would require core to know about the language it should translate to. I don't think this is a very good approach because we would have to make sure we offer the same languages for core and postorius and I'm not sure how we would pass the languages to core...
I propose core to return an error code with each request. That way postorius can add strings for all the errors that it cares about and translate them accordingly. This would require an api change. Since 3.1 would change the api anyway, it would be nice to include the changes there.
What do you think of my proposal? Do you have a better idea? The specifics have to be worked out of course.