Just like you can configure site definitions on a per site (per path) basis it would be a nice improvement if you could do that same within any content app.

For example, let's say I have an articles app. I use this articles app for managing articles for 2 different sites. The different sites are represented in the content app as 2 folders with the same name as the site they hold articles for.

Lets say site1 has en and es configured.
Lets say site2 has de and fr configured.

Now if I want my articles translated in my content app then I would have to configure the fallback definition to have all 4 languages. Then when I open the detail subapp to edit my article the drop down would have all 4 languages even though only 2 of them are valid.

So then my only option is to educate the users. This might be frustrating for users because then you would need to check with site has which languages.