Thanks, Got this fixed by updating the font configuration file as below.
<?xml version='1.0'?>
<!DOCTYPE fontconfig SYSTEM 'fonts.dtd'>
<fontconfig>
<!-- Fallback fonts preference order -->
<alias>
<family>sans-serif</family>
<prefer>
<family>Noto Sans</family>
<family>Open Sans</family>
<family>Droid Sans</family>
<family>Roboto</family>
<family>Tholoth</family>
<family>Noto Sans Sinhala</family>
</prefer>
</alias>
<alias>
<family>serif</family>
<prefer>
<family>Noto Serif</family>
<family>Droid Serif</family>
<family>Roboto Slab</family>
<family>Tholoth</family>
<family>Noto Sans Sinhala</family>
</prefer>
</alias>
<alias>
<family>monospace</family>
<prefer>
<family>Noto Sans Mono</family>
<family>Inconsolata</family>
<family>Droid Sans Mono</family>
<family>Roboto Mono</family>
</prefer>
</alias>
<match target="pattern">
<test name="lang" compare="contains">
<string>si</string>
</test>
<edit name="family" mode="prepend" binding="strong">
<string>Noto Sans Sinhala</string>
</edit>
</match>
<dir>~/.fonts</dir>
</fontconfig>