I've suggested making putting at least a single page in
kdedir()/doc/default/HTML/<appname>/<appname>.html
a requirement for KDE compliance. The application is free to use the directory to store any help data it requires.
Applications that support more than one language would place the other languages inkdedir()/doc/<lang>/HTML/<appname>/<appname>.html with there being one 'lang' directory for each language code as usual. Arranging the files like this would allow links between the help files of two different applications that both support a given language.
I am not 100% happy with the solution I've suggested as it does not allow any way to fall back to the default language if a required translation is not available.