I have made some adjustments to the NavHeader which should remove the need for the localized version. Please take a look at how the newer NavHeader template reacts when traversing within the localized segment, such as /de. I switched Ziele und Funktionen to use the standard NavHeader. As you may notice, the new templates 'Next' and 'Previous' and 'returnTOC' and 'TOC' will contain a list of the appropriate translated words for each language id.
I plan to add further refinements for the rest of the wiki as needed. I had been attempting to solve the challenges of localizing the wiki when a site malfunction seemed to have taken away all my efforts, but I happen to have run across some breadcrumbs that survived. I have also been away from doing much editing or template work on the wiki for quite some time- at least in part due to sorrow over the loss of hard gained success (with a template relating to localized links).
I noticed how you have managed to translate many different things by using "custompagename" and other similar variables that exist in a number of templates. I hope to make further improvements on the wiki so that those changes might not be needed. I have to think a bit more on some wrinkles as they appear.
Some odd template issues
We may have to fly with what we have for the moment. As near as I can tell, the /en pages have some small template-induced errors with the hover link text on the NavHeader but this is due to the translation system's lag time which acts like a cache. Once the system catches up, I will look at it again to verify whether it is still an issue. I hope that everything continues to work properly in Ziele und Funktionen.
I am currently baffled and must be getting hindered by cache somewhere. The one /de page that I switched back to the standard NavHeader continues to work regardless of the modifications I make, while the 'base' page and /en page continue to have the weird bug. The main good news is that it is not a functional problem, only an aesthetic one.