Keep node/user tabs in one language
What you want, as I understand you, is to have a fixed interface language for a certain role. 

You can adjust the user interface text language in admin/config/regional/language/configure. Unfortunately there's no method based on role. You can only use the user method, so users see the interface in their prefered langauge. This setting is for all users, not just for a role.

You may code a new language detection method based on role, but I don't know where you have to start (except checking the i18n module code).