Hello Jean and Richard
It is early morning for me and the coffee is still working its way through.
I know LibreOffice installs no problem under Lion, Snow Leopard and Leopard Mac operating systems. You just get the warning when you first run LibreOffice.
Richard
Is your Mountain Lion an upgrade from Lion or a brand new install? If it an upgrade, then all I can suggest is a downgrade to Lion, install LibreOffice, open and close the program to make sure it works. If everything is OK, trying upgrading to Mountain Lion again.
I know there has been one or two incompatible problems with Mountain Lion which is usual for an operating systems. I cannot test this one because I do not have Mountain Lion installed - my Macs are too old.
Regards
Peter Schofield