If one wants to override (for example) error pages he can override the TwigBundle Exceptions templates.
If we want to make these templates locale-aware we could put them into app/Resources/TwigBundle/translations/ and they will work .. but the translations system seems to have no way to extract or dump them ...

I think translations:[dump|update] should be overrides-aware and search for translations in every standard place a bundle could be overridden

Hope to have been clear in my explanation :)