Back end and back office are English expressions used in French. They mean the same thing in French than in English.

So back end is used in French dev, opposed to front end.

Back office is something else, relative to administrative departments inside a company.

Now, it happens sometimes that someone uses back office in the meaning of back end, and it makes sense because both apply to the part of the entity (company in the first case, software environment in the second one) that makes the mecanism working but remains invisible to users.

Still I can’t find many resources of back office used in the development context, so I wouldn’t use it.