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.
Ok. I solved the challenge for the hover text (for the left) but I've discovered another bit of oddness. The wiki allows the pagename to be localized, BUT, the magicword I am using in the NavHeader template to shift the title below the icons does not work, it does not get localized (seems to take it from the URL). This is undoubtedly the reason for using the custompagename. I want to solve this issue as well, and see if the same translated data could be used for other templates. Currently using the standard NavHeader template without 'custompagename' causes a duplication of the pagename though non-localized. The real solution may be an adjustment to site CSS somehow. It will take a bit of exploration and effort.. so until then I will adjust the 'right' side of the NavHeader template effect to match the left.