diff --git a/backend/user/api/serializer.py b/backend/user/api/serializer.py
index f65dcfd..a16477a 100644
--- a/backend/user/api/serializer.py
+++ b/backend/user/api/serializer.py
@@ -158,6 +158,7 @@ def deep_merge(base, overrides):
 
 
 DEFAULT_PREFERENCES = {
+    'locale': 'en_US',
     'default_empire_uuid': None,
     'default_cx_uuid': None,
     'default_buy_items_from_cx': True,
@@ -181,6 +182,7 @@ class PlanOverrideSerializer(serializers.Serializer):
 class UserPreferenceSerializer(JSONSafeSerializerMixin, serializers.Serializer):
     # Frontend JSON: camelCase, variable name
     # Backend JSON: snake_case, source
+    locale = serializers.CharField(allow_null=True, required=False)
     defaultEmpireUuid = serializers.UUIDField(source='default_empire_uuid', allow_null=True, required=False)
     defaultCXUuid = serializers.UUIDField(source='default_cx_uuid', allow_null=True, required=False)
     defaultBuyItemsFromCX = serializers.BooleanField(source='default_buy_items_from_cx', required=False)