The FreeCAD offline documentation is created with scripts from the FreeCAD-Wiki. It has actual grown to a file size of more than 100 megabyte. These big files are not part of development installers and executables of FreeCAD. But the offline documentation can be installed separately, which is explained here.
The offline documentation has now also made been available in French and Italian. (Check for other languages.) These files are not part of the official installers of FreeCAD but available as separate packages.
A working offline documentation consist of at least two files: "freecad.qhc" the Qt-helpfile-configuration and "freecad.qch" the compressed Qt-helpfile. They are put together in a ZIP-archive.
The helpfiles can be downloaded here: https://github.com/FreeCAD/FreeCAD/releases/tag/0.18_pre
As a future option they can be installed from within FreeCAD with the Addon manager.
The help files do always have the same names: "freecad.qhc" and "freecad.qch". In order to have different version of the helpfiles, these need to be installed in different directories. In case of a manual download, just store the zip-file locally and extract the archive into the wanted directory.
The actual organization of the offline help does allow only one helpfile to be active. It is therefore not possible to have helpfiles in different languages accessible from FreeCAD at the same time.
In order to make another FreeCAD documentation active the following steps have to be applied:
In case your operation system has no Qt-Assistant and it is not delivered with FreeCAD, it will be necessary to install one.