Bug 1047215 - [Gallery/Video]Background Overlays are not translated

This PR incorporates the patch from bug 1038984 as those changes to dialogs.js are required for this patch.
Do we really need to rename the l10n key?

IIRC, we only do that when the meaning of sentence changed. I cannot make sure with this. If you have some information about this, please also email me. Thanks.