Updated PLANS.md, AGENTS.md, and arabic-localization.md to reflect the “foundations now, full translations later” approach and marked progress accordingly.

Implemented localization foundations across backend and frontend (locale settings/middleware, preferred language, i18n wiring, RTL support, minimal Arabic UI strings, Accept-Language).
Added targeted backend and frontend tests plus a risks note for pending full translation coverage.

backend/apps/accounts/tests/test_locale.py

@@ -0,0 +1,29 @@
+import pytest
+from django.urls import reverse
+
+from apps.accounts.models import User
+
+
+@pytest.mark.django_db
+def test_accept_language_sets_content_language_header(client):
+    response = client.post(
+        reverse("otp_request"),
+        {"phone_number": "123", "channel": "sms"},
+        content_type="application/json",
+        HTTP_ACCEPT_LANGUAGE="en",
+    )
+    assert response.headers.get("Content-Language") == "en"
+
+
+@pytest.mark.django_db
+def test_user_preference_overrides_accept_language(client):
+    user = User.objects.create_user(
+        email="locale@example.com",
+        phone_number="+966512345678",
+    )
+    user.preferred_language = "en"
+    user.save(update_fields=["preferred_language"])
+
+    client.force_login(user)
+    response = client.get(reverse("me"), HTTP_ACCEPT_LANGUAGE="ar-sa")
+    assert response.headers.get("Content-Language") == "en"