Do not break invoices if order has no locale

src/pretix/api/serializers/order.py

@@ -514,6 +514,8 @@ class OrderCreateSerializer(I18nAwareModelSerializer):
             if any(errs):
                 raise ValidationError({'positions': errs})
 
+            if validated_data.get('locale', None) is None:
+                validated_data['locale'] = self.context['event'].settings.locale
             order = Order(event=self.context['event'], **validated_data)
             order.set_expires(subevents=[p.get('subevent') for p in positions_data])
             order.total = sum([p['price'] for p in positions_data]) + sum([f['value'] for f in fees_data], Decimal('0.00'))

src/pretix/base/services/invoices.py

@@ -200,10 +200,10 @@ def regenerate_invoice(invoice: Invoice):
 
 
 def generate_invoice(order: Order, trigger_pdf=True):
-    locale = order.event.settings.get('invoice_language')
+    locale = order.event.settings.get('invoice_language', order.event.settings.locale)
     if locale:
         if locale == '__user__':
-            locale = order.locale
+            locale = order.locale or order.event.settings.locale
 
     invoice = Invoice(
         order=order,