I have a client in the US (Illinois) who calls their custom admin panels the "back room" or sometimes "back end". For example, they will say something like "We'll just create the account in the back room". We always work in English. I cannot say I have encountered "back office", but it doesn't sound too awkward. I wouldn't be surprised by an admin panel being called the "back office", although personally I usually use the term "admin panel" since it's quick and less ambiguous.