Campaign Language value in transaction message request

Campaign signup / reportback transactionals:

[x] Campaign Signup
[x] Campaign Reportback

The language of the specific campaign the user signed up for or reported back to defines what language the transactional message should be in.

Also: The country code value for the user needs to be available to dosomething_mbp.module here: https://github.com/DoSomething/phoenix/blob/dev/lib/modules/dosomething/dosomething_mbp/dosomething_mbp.module#L218-L233

~~BLOCKED by~~: Currently dosomething_settings_get_affiliate_country_code() takes care of defining the affiliate value. This issue is blocked by the need for the functionality within the Drupal App to gather the campaign country as well as the user language.

Sample payload:

{
  "activity": "campaign_signup"
  "email": "xyz@geemail.com"
  "uid": "123456789"
  "merge_vars":
    {
     "MEMBER_COUNT": "4 million"
     "FNAME": "Bob"
     "CAMPAIGN_TITLE": "School Insider",
     "CAMPAIGN_LINK": "https://www.dosomething.org/campaigns/school-insider",
     "CALL_TO_ACTION": "Provide tips and information to new students at your school.",
     "STEP_ONE": "Make Your Sheet!",
     "STEP_TWO": "Snap a Pic",
     "STEP_THREE": "Deliver the Sheet"
    }
  "email_template": "mb-campaign-signup-mx"
  "user_language": "CH",
  "mailchimp_list_id": "133qwer456"
  "birthdate": "916876800"
  "subscribed": 1
  "email_tags"
    {
      "MX",
      "drupal_campaign_signup"
      "drupal_campaign_signup_MX";
    }
  "activity_timestamp" 1432086537
  "application_id": "DRUPAL";
}

Related:

DoSomething / legacy-website

Campaign Language value in transaction message request #5036