In docbook one can specify the language used at the top, and the processing pipeline will use this information to pick the correct translations for 'chapter', 'see also', 'index' etc. And for most languages used with docbook, I guess this work just fine. For example a German user can start the document with <book lang="de">, and the document will show up with the correct content with any of the docbook processors. This is not the case for the language I am working with at the moment, Norwegian Bokmål.
For a while, I was confused about which language code to use, because I was unable to find any language code that would work across all tools. I am currently testing dblatex, xmlto, docbook-xsl, and dbtoepub, and they do not handle Norwegian Bokmål the same way. Some of them do not handle it at all.
A bit of background information is probably needed to understand this mess. Norwegian is not one, but two written variants. The variants are Norwegian Nynorsk and Norwegian Bokmål. There are three two letter language codes associated with these languages, Norwegian is 'no', Norwegian Nynorsk is 'nn' and Norwegian Bokmål is 'nb'. Historically the 'no' language code was used for Norwegian Bokmål, but many years ago this was found to be å bad idea, and the recommendation is to use the most specific language code instead, to avoid confusion. In the transition period it is a good idea to make sure 'no' was an alias for 'nb'.
Back to docbook processing tools in Debian. The dblatex tool only understand 'nn'. There are translations for 'no', but not 'nb' (BTS #684391), but due to a bug (BTS #682936) the 'no' language code is not recognised. The docbook-xsl tool chain only recognise 'nn' and 'nb', but not 'no'. The xmlto tool only recognise 'nn' and 'nb', but not 'no'. The end result that there is no language code I can use to get the docbook file working with all of these tools at the same time. :(
The correct solution is to use <book lang="nb">, but it will take time before that will work with all the free software docbook processors. :(
Oh, the joy of well integrated tools. :/