🌐(backend) add Dutch language support to backend

Add Dutch (nl) language configuration to backend to match available frontend
languages. Ensures consistent language options across the entire application.

src/backend/core/migrations/0013_alter_user_language.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.1.8 on 2025-04-22 14:52
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0012_alter_room_access_level'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='user',
+            name='language',
+            field=models.CharField(choices="(('en-us', 'English'), ('fr-fr', 'French'), ('nl-nl', 'Dutch'))", default='en-us', help_text='The language in which the user wants to see the interface.', max_length=10, verbose_name='language'),
+        ),
+    ]