From what I understand about the Autobahn in particular and most of Germany in general, strict observation of lane choice is not optional. It's an immediately ticketable offense. You pass like you mean it, and get back into the correct lane as soon as possible.
I get crotchety on the Austin loop toll roads when dump trucks and service vehicles that are governor limited to 75 mph are on it, usually side by side, in areas marked for 85 mph.