When I install some language pack, which is the _exact_ translation of langenus.xpi, uninstall en-US language pack, and restart Mozilla, I see this message, which takes half of the screen and prevents one from doing anything with Mozilla (see screen-shot). This is due to the reason that not all */locale/* files are included in langenus.xpi (so they are not in the other translation), in other words, en-US locale files are uninstalled with no replacement. Reproducible: Always Steps to Reproduce: 1. Install Mozilla. 2. Install and enable en-GB language pack (langengb.xpi), or any other language pack, which has the same structure and number of files as langenus.xpi. 3. Uninstall en-US language pack via Preferences --- Languages/Content --- Installed Language Packs --- English (US) --- Uninstall. 4. Restart Mozilla. Actual Results: Mozilla is unusable, due to a message that takes half a screen (see screen-shot). Expected Results: Everything works fine, no noticeable changes.

We need not uninstall the specific components of the locale, if there is no replacement in other locales that are installed. Adding xpi-engine@installer.bugs to CC list, since this looks like an uninstall engine bug...

I'm not sure the languages pref panel knows what packages are or aren't in a particular language pack. The real solution will be to implement an XPInstall uninstaller and let that unregister the locales, and get rid of the "remove" button from the languages pref dialog.

If this isn't a recent regression, without a solution in hand, we're not going to block on it.