Select which lang packs should be available to user

[Kristina Hoeppner]

When you have a localized language pack that is based off a standard one, both the standard lang pack and the custom one are displayed and the site admin cannot just display the custom one. It should be possible for the site administrator to decide which lang packs of the installed ones shall be used.

This is not so important for lang packs of entirely different languages because these could just not be installed. However, for small localized changes this is important so that you don't see the standard English lang pack (which you need because the local lang pack is based on it) but only the one with the custom changes, but still have the standard pack available for the rest of the strings.