The OTRS admin manual can be translated via transifex as described in the section on translating OTRS. You can join a language team on transifex to improve an existing translation or even suggest a new language to translate the admin manual to.

It is important that the structure of the generated XML stays intact. So if the original string is "Edit <filename>Kernel/Config.pm</filename>", then the German translation has to be "<filename>Kernel/Config.pm</filename> bearbeiten", keeping the the XML tags intact. Regular < and > signs that are escaped in the source text must also be escaped in the traslations (like "<someone@example.com>"). Scripts and examples usually do not have to be translated (so you can just copy the source text to the translation text field in this case).