It seems to me that flags are usually named in the form Flag of X. This one would be called flag_of_the_german_empire. I think the tagger was going for a shorter, but similar form. I might have chosen german_imperial_flag in that case. On the other hand, I think we mostly use the form national Flag here; in that case maybe german_empire_flag is the weakest of the three terse forms and we should use either imperial_german_flag or german_imperial_flag.
