That menu is generated by \sitecore\shell\Applications\Content Manager\Galleries\Languages\Gallery Languages.xml
and the associated CodeBeside in Sitecore.Shell.Applications.ContentManager.Galleries.Languages.GalleryLanguagesForm, Sitecore.Client
The rendering of each option is control by \sitecore\shell\Applications\Content Manager\Galleries\Languages\Gallery Languages.Option.xml
although the values are set from the CodeBeside.
BTW, I just used the Chrome Dev Tools to figure out what was going on. From the Network Panel there is an XHTTP call to default.aspx?xmlcontrol=Gallery.Languages
. Useful debugging steps.