🐛(backend) fix flaky test after add fr translations

Define user language to test invitation email content.

src/backend/core/models.py

@@ -21,8 +21,8 @@ from django.core.exceptions import ValidationError
 from django.db import models, transaction
 from django.template.loader import render_to_string
 from django.utils import timezone
+from django.utils.translation import gettext, override
 from django.utils.translation import gettext_lazy as _
-from django.utils.translation import override
 
 import jsonschema
 from timezone_field import TimeZoneField
@@ -961,14 +961,15 @@ class Invitation(BaseModel):
         """Email invitation to the user."""
         try:
             with override(self.issuer.language):
+                subject = gettext("Invitation to join La Régie!")
                 template_vars = {
-                    "title": _("Invitation to join La Régie!"),
+                    "title": subject,
                     "site": Site.objects.get_current(),
                 }
                 msg_html = render_to_string("mail/html/invitation.html", template_vars)
                 msg_plain = render_to_string("mail/text/invitation.txt", template_vars)
                 mail.send_mail(
-                    _("Invitation to join La Régie!"),
+                    subject,
                     msg_plain,
                     settings.EMAIL_FROM,
                     [self.email],

src/backend/core/tests/test_models_invitations.py

@@ -248,7 +248,9 @@ def test_models_team_invitations_email():
     assert len(mail.outbox) == 0
 
     factories.TeamAccessFactory(team=team)
-    invitation = factories.InvitationFactory(team=team, email="john@people.com")
+    invitation = factories.InvitationFactory(
+        team=team, email="john@people.com", issuer__language="fr-fr"
+    )
 
     # pylint: disable-next=no-member
     assert len(mail.outbox) == 1
@@ -257,10 +259,10 @@ def test_models_team_invitations_email():
     email = mail.outbox[0]
 
     assert email.to == [invitation.email]
-    assert email.subject == "Invitation to join La Régie!"
+    assert email.subject == "Invitation à rejoindre La Régie!"
 
     email_content = " ".join(email.body.split())
-    assert "Invitation to join La Régie!" in email_content
+    assert "Invitation à rejoindre La Régie!" in email_content
     assert "[//example.com]" in email_content