Is there any way to get them all to use one app setting?
Maybe R2 could be your answer here. I wrote a post about new property types, not tried it for multi-lingual yet but guessing they will always use the custom settings value regardless of langauge:
http://world.episerver.com/Blogs/David-Knipe/Dates/2010/12/New-property-types-in-EPiServer-6-CMS-R2-Beta/
Is there any way to get them all to use one app setting?