In nwnplayer.ini under game options.
ClientChatLogging=0
Setting this to 1 will add all player chat heard by the client to the client log. The file can be found in the "logs" directory of the NWN base install path (eg. nwnclientLog1.txt). Note that the log will overwrite itself; preventing long term chat logging over multiple play sessions. This can be prevented by renaming the log after every session, or using a Log Rotator
[1].