In setup PHP gettext should be optional - removed from "step 2. Check of pre-requisites" module list. It is not required. System works without it, only translations will not work.
Due to this, frontend messages should also be then removed, but not all. Only place to show error message should be where we select language - in profiles.
We shouldn't be allowed change or select different language, if gettext module is disabled. Probably in dropdown they can be marked as disabled and/or maked as red.