Once phpBB languages are made I doubt they'd need to be edited much but I suppose it could encourage a few more translations (how many languages are already done?) if there was an export feature. *shrug*
The translated german language files have some errors and some sentences are complicated somehow, because they seem to be performed with google translate and are not really formed liked germans speak. So it really would be useful to change something within phpBB instead of using external programs.
Until now I thought there only can READY MADE language packs be installed and deleted and not can't be edited.
But you are right. After clicking the language name I can see and edit. But I have a problem to save that. I don't like so save it with FTP anywhere else, only saving directly in the language directory of the server, where the forum is installed, to use it there.
So I think that isn't so comfortable like it should and could be.
As I'm using the phpbb framework also for websites I developed a language copy script.
It uses a master language pack and copies the language variable of a second language pack in new files named the same as the master.
All pack should be in the store folder.