Noto Sans Bengali does not contain glyphs for the Latin alphabet.
You may have thought it did if working in a word processor on Windows, because such programs may silently substitute other fonts when glyphs are missing. harftex gives you more control and, in consequence, requires you to say exactly what you want.
Here’s one way to do that:
\documentclass{article}
\usepackage{harfload,fontspec}
\setmainfont{Noto Sans Bengali}[RawFeature={mode=harf}]
\newfontfamily\british{Noto Sans}[Script=Latin]
\parindent 0pt
\begin{document}
বাংলা \textbf{বাংলা}\\
\british English
\end{document}

UPDATE
As of November 2019, you can install luahbtex and replace the lines
\usepackage{harfload,fontspec}
\setmainfont{Noto Sans Bengali}[RawFeature={mode=harf}]
with these:
\usepackage{fontspec}
\setmainfont{Noto Sans Bengali}[Renderer=Harfbuzz,Script=Bengali]
See Ulrike Fischer’s explanation for details.
polyglossia. – Ruixi Zhang Jun 20 '19 at 16:26