marketing/civicamp-calgary#3 - receipt follows preferred_language

Created by: samuelsov

See https://lab.civicrm.org/marketing/civicamp-calgary/issues/3

The receipt is printed in the user preferred language by default - it uses the following rule :

1. the preferred_language if it's defined and is English or French (any "en" or "fr" short code)
2. the default site language if it is English or French (any "en" or "fr" short code)
3. fallback to en_US