Better method for finding new language key

[pak21]

A slightly nicer method for finding the English localizations in the latest patch - Arena has moved the value we're looking for from the langkey field to the isoCode field, I'm guessing to allow for (e.g.) different US English vs British English translations in future.

If we're not bothered about backwards compatibility, we could just drop the langkey check. I really doubt many people run the latest Python on an older Arena install but you never know...