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 XML tags intact. Regular < and > signs that are escaped in the source text must also be escaped in the translations (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).