Overlay does not allow language switching

Steps to reproduce:

• Enable the Locale and the Overlay module
• Add a new Language on Languages Configuration page (e.g. German)
• Edit both languages and either
  • Set language domains (e.g. "http://drupal7.dev" and "http://de.drupal7.dev")
  • Or, set language prefixes (e.g. "" and "de")
• Enable URL detection on the Detection & Selection page, configure URL detection to use "Domain" or "Path prefix" respectively.
• To be able to tell the interface language, import translations for the 2nd language (e.g. drupal-7.0-alpha6.de.po)
• Enable the "Language Block" (e.g. add it to the "content" region)
• Navigate to some admin page and try to switch the language using the language switcher...
  • With language domains the interface language will stay the same as it was
  • With language prefixes and the patch in #759844: Overlay does not work with prefixed URL paths applied the overlay is removed
• (In either case if you disable the overlay module -- or try switching language outside the overlay -- then language switching works.)

The priority for this is up for discussion considering that probably not an awful lot of admin users need to switch their interface language all the time...

Some notes:

• This will probably also affects modules like the domain module which - as far as I remember - depends on being able to switch domains for configuration.
• #284625: Non-localized links in translation table... would make way more sense if language switching while using the overlay module works.