You should have something like this in your webalizer.conf to make webalizer resolve the countries:
# DNSCache specifies the DNS cache filename to use for reverse DNS lookups.
# This file must be specified if you wish to perform name lookups on any IP
# addresses found in the log file. If an absolute path is not given as
# part of the filename (ie: starts with a leading '/'), then the name is
# relative to the default output directory. See the DNS.README file for
# additional information.
# DNSChildren allows you to specify how many "children" processes are
# run to perform DNS lookups to create or update the DNS cache file.
# If a number is specified, the DNS cache file will be created/updated
# each time the Webalizer is run, immediately prior to normal processing,
# by running the specified number of "children" processes to perform
# DNS lookups. If used, the DNS cache filename MUST be specified as
# well. The default value is zero (0), which disables DNS cache file
# creation/updates at run time. The number of children processes to
# run may be anywhere from 1 to 100, however a large number may effect
# normal system operations. Reasonable values should be between 5 and
# 20. See the DNS.README file for additional information.