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This should not be able to happen but could it be that someone has disabled that language in adminmode? Or change language access rights? Episerver do not delete a specific language so it feels like there is an human error otherwise other language shoud be affected as well. Have you checked the changelog in episerver as well? In admin you can get a lot of information in the changelog. Not sure this will show but could be worh trying.
Our Dutch production site which uses EpiServer CMS 7 has a 3 three language branches, of which Dutch is the main language. All of a sudden 1 language branch (german) of the start page was missing. The 'Versions' widget in EpiServer did not list this german version as 'deleted' and didn't show a version of this language branch at all.
To fix this, I had to translate and setup the entire homepagee again. Also, because this startpage was used as an entry point for the www.*.de domain the entire german site stopped working.
Our logs don't show any weird activity that might cause this page to dissapear so I wonder what I can do to prevent this kind of error in the future?