Contact preferred language should be nullable

See also CRM-6942 and CRM-7119, there is a precedent to make preferred language non-nullable, and to default to the site language. To preserve this behavior where it's desired, there should be a configuration option which toggles nullable behavior.

IMO this is the wrong approach, a user's language should be considered undetermined unless explicitly entered. We can always default to sending communications in the site language, but we are losing information about the contact by setting a default language on their record when we do not actually know their language preferences for certain. NULL should be a thing.