Make sure documentation present in both --help and --man output!
Is there demand for this?
I haven't seen demand from any individual person, but I think it's not too hard to do, and it makes using Ack a little easier for speakers of non-US English.
As a non-US English speaker I am not too bothered either way. I am comfortable that I store my favourite URLs as a favorite and that licence is spelt license.
On balance I think have the british spelling just makes for option bloat. What if ACK_PAGER_COLOR and ACK_PAGER_COLOUR are both defined but different?
I don't want to do this. It's more stuff to maintain and nobody has requested it.