Fix duplicate locale defintion in TranslatableCreateView template (#580)

* For Wagtail versions below 3.0, the locale must be added at this point.
* Extended "test_create_view" to also ensure a forrect form action URL

Co-authored-by: benmth <b@omegabyte.lu>

wagtail_localize/modeladmin/templates/wagtail_localize/modeladmin/translatable_create.html

@@ -1,7 +1,6 @@
 {% extends "modeladmin/create.html" %}
 
 {% block header %}
-  {% include "wagtailadmin/shared/header_with_locale_selector.html" with title=view.get_page_title subtitle=view.get_page_subtitle icon=view.header_icon tabbed=1 merged=1 %}
+    {% include "wagtailadmin/shared/header_with_locale_selector.html" with title=view.get_page_title subtitle=view.get_page_subtitle icon=view.header_icon tabbed=1 merged=1 %}
 {% endblock %}
-
-{% block form_action %}{{ view.create_url }}?locale={{ locale.language_code }}{% endblock %}
+{% block form_action %}{{ view.create_url }}{% if wagtail_version < "3.0" %}?locale={{ locale.language_code }}{% endif %}{% endblock %}

wagtail_localize/modeladmin/tests.py

@@ -78,6 +78,13 @@ def test_create_view(self):
         create_url = reverse("wagtail_localize_test_testmodel_modeladmin_create")
         response = self.client.get(create_url)
 
+        # Check for correct Form Action
+        self.assertContains(
+            response,
+            '<form action="/admin/wagtail_localize_test/testmodel/create/?locale=en"',
+        )
+
+        # Check, if other Language is available in Dropdown
         self.assertContains(
             response,
             '<a href="/admin/wagtail_localize_test/testmodel/create/?locale=de"',