use translations for missing params

app/controllers/discourse_donations/charges_controller.rb

@@ -10,12 +10,10 @@ module DiscourseDonations
       output = { 'messages' => [], 'rewards' => [] }
 
       if create_account
-        if (email.nil? || email.empty?)
-          output['messages'] << 'Please enter your email address'
+        if !email.present? || params[:username].nil?
+          output['messages'] << I18n.t('login.missing_user_field')
         end
-        if params[:username].nil?
-          output['messages'] << 'Please enter a username'
-        elsif ::User.reserved_username?(params[:username])
+        if params[:username].present? && ::User.reserved_username?(params[:username])
           output['messages'] << I18n.t('login.reserved_username')
         end
       end

spec/controllers/discourse_donations/charges_controller_spec.rb

@@ -20,7 +20,7 @@ module DiscourseDonations
 
     it 'expects a username if accounts are being created' do
       post :create, { email: 'zipitydoodah@example.com', create_account: 'true' }
-      expect(body['messages']).to include('Please enter a username')
+      expect(body['messages']).to include(I18n.t('login.missing_user_field'))
       expect(response).to have_http_status(200)
     end
 
@@ -34,12 +34,12 @@ module DiscourseDonations
     describe 'new user' do
       it 'has a message when the email is empty' do
         post :create, { create_account: 'true', email: '' }
-        expect(body['messages']).to include('Please enter your email address')
+        expect(body['messages']).to include(I18n.t('login.missing_user_field'))
       end
 
       it 'has a message when the email is empty' do
         post :create, { create_account: 'true' }
-        expect(body['messages']).to include('Please enter your email address')
+        expect(body['messages']).to include(I18n.t('login.missing_user_field'))
       end
 
       it 'has a message when the username is reserved' do