Hi,
because of the fact that Localization is a singleton package and can therefore just have only one instance you have to create an own locale for each subsite.
In /acs-lang/admin you have the link "Create New Locale" at the bottom of the page.
I've never tried that out so I cannot guarantee that it works perfectly but it would be the methodology I would try at first (if somebody does not come up with a better idea..)
