In fact it was faster than I thought
typing the syntax "apt-get install manpages-fr" in the console LibreELEC:~ # I got the answer immediately (after translation of the text into French ...)
There is no working 'apt-get'.
'apt-get' is a command to install, update and remove software which
is stored in a non local repo. 'apt-get' does nothing then connecting to such
repo, downloads the software, unpacks the software, updates a big
local database with all filepaths and other informations about the
installed software or removes or updates installed Software.
With LibreELEC it is not possible to change the system for security and
stability reasons so even 'apt-get' would not be able to do this.
We also dont have and want to maintain such a repo for various other
great reasons.
Also Ubuntu or Debian packages are often outdated and not compatible
with LibreELEC
TIP: use XBMC's addon browser to enhance your LibreELEC system
So I'm going to see XBMC