The german translation is in openclonk\docs\de.po

The documentation you linked to is actually mostly relevant for someone wanting to do translations :-)

For just writing, edit one of the .xml files in docs/, and open them with a web browser to check. There isn't any documentation on the xml vocabulary, but it's  a straightforward mix of tags copied from HTML and mostly self-explanatory custom tags. Firefox unfortunately won't render the bare xml files correctly when read directly from the file system, so one needs to use a local web server. But on windows, Internet Explorer uses local file system permission checks that allow the xml files to correctly render directly in the browser.