Fix steps for adding translations to an existin snippet

* Fix BootstrapTranslatableMixin to skip TranslatableModel's system check for the missing unique constraint
* Fix incorrect import for BootstrapTranslatableModel and use of `--empty` in final schema migration
* Clarify handling of Meta classes, the fact that you need to run makemigrations for step 1, and what to do for the non-nullable 'locale' prompt

docs/advanced_topics/i18n.rst

@@ -508,6 +508,13 @@ This will add the two fields without any constraints:
     class Advert(BootstrapTranslatableMixin, models.Model):
         name = models.CharField(max_length=255)
 
+        # if the model has a Meta class, ensure it inherits from
+        # BootstrapTranslatableMixin.Meta too
+        class Meta(BootstrapTranslatableMixin.Meta):
+            verbose_name = 'adverts'
+
+Run ``python manage.py makemigrations myapp`` to generate the schema migration.
+
 Step 2: Create a data migration
 *******************************
 
@@ -524,7 +531,7 @@ in that app:
 .. code-block:: python
 
     from django.db import migrations
-    from wagtail_localize.bootstrap import BootstrapTranslatableModel
+    from wagtail.core.models import BootstrapTranslatableModel
 
     class Migration(migrations.Migration):
         dependencies = [
@@ -556,6 +563,9 @@ constraints:
     class Advert(TranslatableMixin, models.Model):  # Change this line
         name = models.CharField(max_length=255)
 
+        class Meta(TranslatableMixin.Meta):  # Change this line, if present
+            verbose_name = 'adverts'
+
 Step 4: Run ``makemigrations`` to generate schema migrations, then migrate!
 ***************************************************************************
 
@@ -565,9 +575,14 @@ migrations:
 
 .. code-block:: bash
 
-    python manage.py makemigrations myapp --empty
+    python manage.py makemigrations myapp
     python manage.py migrate
 
+When prompted to select a fix for the nullable field 'locale' being changed to
+non-nullable, select the option "Ignore for now" (as this has been handled by the
+data migration).
+
+
 Translation workflow
 --------------------
 

wagtail/core/models.py

@@ -574,6 +574,11 @@ class BootstrapTranslatableMixin(TranslatableMixin):
         Locale, on_delete=models.PROTECT, null=True, related_name="+", editable=False
     )
 
+    @classmethod
+    def check(cls, **kwargs):
+        # skip the check in TranslatableMixin that enforces the unique-together constraint
+        return super(TranslatableMixin, cls).check(**kwargs)
+
     class Meta:
         abstract = True