Is it possible to switch languages automatically without tags, in Polyglossia?

Seems like you are using XeLaTeX. If so, as @kongo09 mentions in this question, a package called ucharclasses works. Here is the MWE:

documentclass[xelatex]{article}
usepackage{fontspec}
newfontfamily{defaultfont}{Times New Roman}
newfontfamily{latinfont}{Times New Roman}
newfontfamily{devanagarifont}{NotoSansDevanagari-Regular}

usepackage[Latin, Devanagari]{ucharclasses}
setDefaultTransitions{defaultfont}{}
setTransitionsForLatin{latinfont}{}
setTransitionTo{Devanagari}{devanagarifont}

begin{document}

The ending हल् letter of an उपदेश is called 'इत्'.

end{document}

It is possible to do this with babel. This new preamble should work with your document, so long as you used textenglish and begin{sanskrit} instead of devanagarifont. It requires luahbtex (lualatex in TeX Live 2020 works), but has a bug with xetex (as of July 2020).

documentclass{article}
tracinglostchars=2
usepackage{iftex} % For ifluahbtex, ifxetex, etc.
usepackage[paperwidth=10cm]{geometry} % Format the MWR for TeX.SX
pagestyle{empty}

ifluahbtex
  usepackage[bidi=basic,main=english]{babel}
  usepackage{fontspec}
  defaultfontfeatures{ Renderer=HarfBuzz,
                        Scale=MatchLowercase,
                        Ligatures=TeX }
  usepackage{microtype}
else
  % The only other engine that works, as of 2020, is XeTeX.
  usepackage[bidi=default,main=english]{babel}
  usepackage{fontspec}
  defaultfontfeatures{ Scale=MatchLowercase,
                        Ligatures=TeX }
fi

babelprovide[onchar=ids fonts]{sanskrit-devanagari}

babelfont{rm}
          [Ligatures=Common, Scale=1.0,Language=Default]{Noto Serif}
babelfont{sf}
          [Ligatures=Common,Language=Default]{Noto Sans}
babelfont[sanskrit-devanagari]{rm}
          {Noto Serif Devanagari}
babelfont[sanskrit-devanagari]{sf}
          [Language=Default]{Noto Sans Devanagari}
babelfont{tt}
          [Language=Default]{Noto Sans Devanagari UI}

babeltags{sanskrit=sanskrit-devanagari} % Add textsanskrit, begin{sanskrit}
babeltags{english=english} % Add textenglish, begin{englisn}

begin{document}
sffamily
The ending हल्  letter of  an उपदेश is called `इत्'.
end{document}

Recall that quotes in TeX are

`foo ``bar" baz' or ‘foo “bar” baz’