Adding en_GB locale

UK uses a different date structure then American English ('day, month' vs
'month, day'), so we added the `en_GB` locale with the dates appropriately
changed.

We noticed that some .po files use `X-Is-Fallback-For`, for instance in the
`pt` locale you have:

    "X-Is-Fallback-For: pt-ao pt-cv pt-gw pt-mz pt-st pt-pt\n"

As far as we tried it out, adding a similar line to the `en` locale didn't
change anything (the translations from `en` are automatically taken if they do
not exist in `en_GB` even if the `X-Is-Fallback-For` directive isn't included).
If it would be appropriate to add it, drop a comment.