I am new to LibreOffice.
How do I add Hebrew support for LibreOffice on windows 10 or Ubuntu
Activate the Complex Text Layout (CTL) option in Tools → Options → Language Settings → Languages, Default Languages for Documents, and select Hebrew from its language list.
Also, if you want to have the user interface in Hebrew, you need to install a Hebrew language pack on Ubuntu (on Windows, all UI languages are available with the default installation). If you want offline help in Hebrew, you need to install a help pack in the both systems.