How can I use lang files for content of my guidebook?
I don't found in wiki.

I don't support using translation keys for content, yet. I implemented translations by having whole books in different langauges. Like, bookname.es_es.xml, bookname.en_gb.xml and such.
I can implement using translation keys, if people have an use for them, or prefer using that to populate the book contents...

It is suitable, thanks!