don't fail when english is not available

Django runs all management commands with 'en-us' as the activated language.
If 'en' or one of the en-sublanguages is not defined in CMS_LANGUAGES,
``update_site_and_page_choices`` fails.

I'm not 100% sure if this fixes the problem at the right level, though...
Merge pull request #1742 from piquadrat/feature/fix-no-english

don't fail when english is not available