Fix localization

I think that I finally managed to fix the localization stuff.
Now everything should be fine: callink Localization::changeLocale loads the core + package + site language files.
BTW, we've got some breaking change here. IMHO they're quite minor, but a feedback from the core team is needed:
1. I removed Localization::addSiteInterfaceLanguage: it's useless since changeLocale does everything that's needed
2. Package::setupPackageLocalization had an unused `$key` parameter. I replaced it with the `$translate` parameter that's needed by Localization::changeLocale.