This minor issue came up as some users wanted solely the administrative menu (navbar) rendered output to reflect the chosen user language preference. This is different behaviour then what can be accomplished by going to the admin/config/regional/language/configure screen and setting the following (with precedence)
User interface text language detection
Content language detection
As much more then just the admin menu rendered output would be linked to the user preference such as breadcrumbs, menu blocks etc.
I have added a small patch that adds a checkbox to the user screen that when toggled will tell navbar to follow the user preference.
I have no expectations that this will get in but any insights / feedback is welcome. I was hoping for a more practical way but had to settle for just wrapping logic around the hook_invocation for navbar under navbar_pre_render(). Everything does seem to function as expected though albeit with limited testing. ^_^
Thanks so much!