Validate email_language on registration

app/forms/register_user_email_form.rb

@@ -4,6 +4,7 @@ class RegisterUserEmailForm
   include FormEmailValidator
 
   validate :validate_terms_accepted
+  validates_inclusion_of :email_language, in: I18n.available_locales.map(&:to_s).append(nil)
 
   attr_reader :email_address, :terms_accepted
   attr_accessor :email_language

spec/forms/register_user_email_form_spec.rb

@@ -240,5 +240,26 @@
         expect(submit_form.errors).to eq errors
       end
     end
+
+    context 'when user provides invalid email_language' do
+      it 'returns failure with errors' do
+        errors = { email_language: [t('errors.messages.inclusion')] }
+        extra = {
+          domain_name: 'gmail.com',
+          email_already_exists: false,
+          throttled: false,
+          user_id: 'anonymous-uuid',
+        }
+        submit_form = subject.submit(
+          email: '[email protected]',
+          terms_accepted: '1',
+          email_language: '01234567890'
+        )
+
+        expect(submit_form.success?).to eq false
+        expect(submit_form.extra).to eq extra
+        expect(submit_form.errors).to eq errors
+      end
+    end
   end
 end