\documentclass{article}

\newcommand\remap[2]{%
  \catcode`#1=\active
  \begingroup
  \lccode`~=`#1%
  \lowercase{\endgroup\def~{\char`#2}}\ignorespaces}
\newcommand\remapchars{%
  \begingroup
  \remap देp
  \remap वq
  \remap नाr
  \remap गs
  \remap रीt
  \innerremap}
\newcommand\innerremap[1]{#1\endgroup}

\begin{document}

`abcd'\remapchars{`देवनागरी'}`abcd'

\end{document}