I purchased the North American English version of AppleShare IP 6.1, but when I try to install Mac OS 8.5 from the included disc, it asks me whether I want the British, Australian, or International English version installed.

The international version of the Mac OS 8.5 disc ships with AppleShare IP 6.1. The part number of this disc is ZU691-2157-A. Simply install the International English (Z) version. Any of the three will work with AppleShare IP 6.1. If one of the other two versions are installed already, there is no functional need to go back and reinstall.