Hi!
Certainly having English as the default UI is a good thing, but it does not help a seven year old French girl to find a picture of a "cheval".
Example of commons is really worst you could pick. First of all, we shouldn't use mediawiki for image repository at all ;-)
So it does cost but is there an alternative, an alternative that provides us with the required functionality ??
I did hear about technology in the works, that will make all translations magic - puff, it is all in there. No, there is no other technology. Hence it is question of functionality, sometimes.