Thanks ICTA Siyabas
1 - First step is to type the address bar "about:config" and press go
2 - "Power-User Settings" where some settings related to loading of pages will be shown, from this menu we have to change the setting "Use bitmap fonts for complex scripts". Default selection for this setting will be No. Inorder to read Tamil pages, change this setting to Yes. (Select "BITMAP IMAGE" option YES or NO to select "YES" & Save the Settings)
3 - Now click save and go back to the Opera Mini home to open any Tamil & Sinhala page and see.


