I18N OnePage

[pikku]

If you use the I18N plugin, you can mix normal pages and pages combining multiple pages:

• create a template like described above and use set_onepage_content(return_page_slug())
  
• create an empty page "my-multipage" and assign it this template
  
• create other pages as children of this page and add them to the menu.
  

If you display "my-multipage", the content of all its children will be displayed on the page.

Hello,
i want to create a onepager with multiple pages and one or two additional normal pages (credits, legal stuff).
So I installed I18n, I18base, and this I18N onepage plug in. After editing my template as described everything worked fine for the multipages.
Then I altered the code to set_onepage_content(return_page_slug()), created the empty page (even called it my-multipage at first, then changed to index) with subpages + one extra page.
Again, the multipages work, but the normal page is not showing up in the menu!?
What am I doing wrong?
I played arround a little and assigned a template with set_onepage_content() to the single page - didn`t help.
Then I recognized, if I add a second i18n menu call after the onepage menu call, I get the menu link of the extra page, but the page is empty - I guess because of the onepage content before get_header?
The same happens, when hardcoding this single menu link.
The only way to get the menu link AND the content was to set up a second empty parent page with the single normal page as a child.
But then I still have a problem: on the multipages all the menu links are showing up (this child pages, the second parent, showing its child and the hardcoded link as well), but on all this other pages the onepage menu is missing. (Even on the second parent and child page - the only difference here is, the 1st parent is the index slug)

I relly can´t see the solution, and as there´s not even showing up the content of the normal page with all the considered settings I didn`t try to follow aldebarans challenging suggestions.

I hope somebody can help me!?