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

Question

I want to type text that may contain both Devanagari or Roman script, and I don't want to change language mid sentence every time. Is it possible to merge two fonts, one which supports Devanagari glyphs, and one which supports Roman glyphs into one language?

Currently, I am doing

\setdefaultlanguage{sanskrit}
\newfontfamily\sanskritfont[Script=Devanagari]{NotoSansDevanagari-Regular}

And since NotoSansDevangari-Regular does not have glyphs for roman characters, it renders the line

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

as

��� ������ हल् ������ �� �� उपदेश �� ������ 'इत्'.

Is it possible to somehow add NotoSans-Regular alongwith NotoSansDevanagari-Regular in the language so that it can render both scripts properly without having to explicitly do \begin{english}\end{english} ?

Answer 1

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’

Answer 2

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}