Use django-i18nfield library (#418)

src/pretix/base/services/invoices.py

@@ -10,6 +10,7 @@ from django.db import transaction
 from django.utils.formats import date_format, localize
 from django.utils.timezone import now
 from django.utils.translation import pgettext, ugettext as _
+from i18nfield.strings import LazyI18nString
 from reportlab.lib import pagesizes
 from reportlab.lib.styles import ParagraphStyle, StyleSheet1
 from reportlab.lib.units import mm
@@ -21,7 +22,7 @@ from reportlab.platypus import (
     Table, TableStyle,
 )
 
-from pretix.base.i18n import LazyI18nString, language
+from pretix.base.i18n import language
 from pretix.base.models import Invoice, InvoiceAddress, InvoiceLine, Order
 from pretix.base.services.async import TransactionAwareTask
 from pretix.base.signals import register_payment_providers

src/pretix/base/services/mail.py

@@ -8,9 +8,10 @@ from django.conf import settings
 from django.core.mail import EmailMultiAlternatives, get_connection
 from django.template.loader import get_template
 from django.utils.translation import ugettext as _
+from i18nfield.strings import LazyI18nString
 from inlinestyler.utils import inline_css
 
-from pretix.base.i18n import LazyI18nString, language
+from pretix.base.i18n import language
 from pretix.base.models import Event, InvoiceAddress, Order
 from pretix.celery_app import app
 from pretix.multidomain.urlreverse import build_absolute_uri