Please refer to the questions I have asked here and the answer which solved my problem.
I have encountered another difficulty. I have used the same code suggested by Thérès here with a different font namely Kadwa (github.com/huertatipografica/Kadwa/tree/master/…).
It contents the glyphs (uni0932094D0932.loclMAR and uni0932094D0932.loclMAR) which is localized variants for ल्ल and ल्प respectively.
Now with the code I could get local variants for श and ल, but even though the localized variants for ल्ल and ल्प have same language code mentioned, they are not rendered properly
The code and result image is provides here.
\documentclass{article}
\usepackage{fontspec,polyglossia}
\setdefaultlanguage{Sanskrit}% https://tex.stackexchange.com/a/382981
\setmainfont{NotoSerifDevanagari-Regular.ttf} [Script=Devanagari,Mapping=devanagarinumerals]
\newfontfamily\Kadwaone[Script=Devanagari,Mapping=devanagarinumerals]{Kadwa}
\newfontfamily \Kadwatwo[Script=Devanagari,Language=Marathi,Mapping=devanagarinumerals]{Kadwa}% note the addition of Language=Marathi
\begin{document}
{\Kadwaone ५, ८, श, ल, क्क, ल्ल, प्ल}
\bigskip
{\Kadwatwo ५, ८, श, ल, क्क, ल्ल, प्ल}
\end{document}
