dos not support sum impotent fonts only. so can’t use with ‘sinhala’ language
You have to set up your operating system. You need a suitable Unicode font and a keyboard driver. That is not the task of LibreOffice. You might find what you need on Local Languages Initiative | ශ්රී ලංකා තොරතුරු හා සන්නිවේදන තාක්ෂණ නියෝජිතායතනය - ICTA. It has a read button “FONTS” and a read button “KEYBOARD INPUT & ETC”.