Great idea, but I propose a slightly different structure (similar to second method in david's post + fallback):
- styles/prosilver/lang/en.php
- styles/prosilver/lang/de.php
- styles/prosilver/lang/default.php
Why only one directory?
Few suggestions here put files in sub directories. There is only 1 file per language pack, no need to make multiple directories.
Why not store it with theme?
I think mixing php files with images and css is a bad idea. Nether template nor theme should not have any php files.
What is "default.php"?
Its a fallback php file that includes default (English) language pack. When including style specific language pack, if directory styles/{style}/lang exists, but there is no php file for current language pack, phpBB should include default.php. This way if style doesn't have language pack for current language, style will still work properly.