Can you explain to me a bit about the backup. I do not understand so much in the translate wiki technique :)
See Translatewiki.net_languages#Fallback_language_(MediaWiki) if you don't know what's fallback languages.
You can use serbian language. (sr-el if you want only latin)..
I think we don't need fires and furies on this topic. If you don't want other fallback languages, just say that, we use English as default fallback settings as mostly TWN is translating messages from English to languages that you use.