Mario, your question "why ajax" I think I understood correctly now only. With the language switch I had the question where to start in the at the interaction. A common approach would be GET() in the corresponding app. But the app Lang would be too late in this use case. The starting point would be the app menu. However, I have not found a suitable entry point in it that is available on every page of the interaction. There is no usable "anchor" in the templates for this. That's why I chose POST() of the Lang app as entry point, and not GET(), because with the logic of immediate language switching GET() can't be relevant at all. Also INIT() is not suitable. Hopefully I have described this understandably.