fix(i18n): normalize setlang next path server-side

mandelstudio/i18n_utils.py

@@ -0,0 +1,31 @@
+from __future__ import annotations
+
+import re
+from urllib.parse import urlsplit, urlunsplit
+
+from django.conf import settings
+
+
+def normalize_set_language_next(value: str | None) -> str:
+    """
+    Normalize the `next` path used by Django's set_language view.
+
+    Removes any leading language prefix from the path so switching from one
+    locale to another cannot produce duplicated prefixes like `/de/en/...`.
+    """
+    if not value:
+        return "/"
+
+    parsed = urlsplit(str(value))
+    path = parsed.path or "/"
+    if not path.startswith("/"):
+        path = f"/{path}"
+
+    language_codes = [code for code, _ in settings.LANGUAGES]
+    if language_codes:
+        pattern = (
+            rf"^/(?:{'|'.join(re.escape(code) for code in language_codes)})(?=/|$)"
+        )
+        path = re.sub(pattern, "", path, count=1) or "/"
+
+    return urlunsplit(("", "", path, parsed.query, ""))