Only set up default locale on FixMyStreet install.

FixMyStreet uses the default locale in order to have working URIs with
floating point numbers. This could be moved to the FixMyStreet install
script with its next release and then removed from here.
All `generate_locales` does is install `language-pack-en` (if present, so Ubuntu) and add an `en_GB` locale. I don't think it does anything that prevents making locales available to any system.