I suggest that localization can be turned on and off by the user. The main advantage is that foreign users can decide whether they want to use the localized environment (which might be not complete), or whether they want to use the English (complete) environment. Also this would allow to update language packs separately from the other components, which might be useful.
This requires that localization fragments get their own "feature" in the eclipse sense. This should work analogous to the Eclipse 3.0.1 languag pack feature.