modified: app.py

modified:   templates/base.html
	modified:   templates/index.html

app.py

@@ -29,6 +29,14 @@ def _set_language():
     g.lang = lang
     g.T = TRANSLATIONS[lang]
 
+@app.after_request
+def _persist_language_cookie(response):
+    """If ?lang= was in the URL, save it as a cookie so it survives navigation."""
+    url_lang = request.args.get("lang")
+    if url_lang in TRANSLATIONS:
+        response.set_cookie("lang", url_lang, max_age=60*60*24*365, samesite="Lax")
+    return response
+
 @app.context_processor
 def _inject_i18n():
     return {"T": g.get("T", TRANSLATIONS["de"]), "lang": g.get("lang", "de")}