It seems like numoffset
is the parameter we would want to manipulate in order to keep the number at (but not in) the right margin, but neither positive nor negative values for that make any difference in your MWE.
If you set textoffset
to -\textwidth
, the number will be aligned with the left margin. Wider labels are handled correctly. However, this breaks expex
's line-breaking algorithm, so it's probably not a workable solution, and depending on the language of the main text, you might not want the numbers on the left anyway.

\documentclass{book}
\usepackage[a4paper,left=2cm,right=2cm,top=2cm,bottom=2cm]{geometry}
\usepackage{expex,polyglossia,lipsum}
\lingset{textoffset=-\textwidth}
\usepackage{showframe} % used to show lines at margins
\setdefaultlanguage[variant=american]{english}
\setotherlanguage[numerals=eastern]{farsi}
\newfontfamily\englishfont[Mapping=tex-text]{Times New Roman}
\newfontfamily\farsifont[Script=arab,Scale=MatchLowercase]{Scheherazade}
\begin{document}
\lipsum[1]
\begin{farsi}
\ex[everyglb=\englishfont] \begingl
\gla این مثال من است//
\glb in masale man ast //
\glb this example me is //
\endgl\xe
\end{farsi}
\excnt 99999\relax % to create a wider label
\begin{farsi}
\ex[everyglb=\englishfont] \begingl
\gla این مثال من است//
\glb in masale man ast //
\glb this example me is //
\endgl\xe
\end{farsi}
% long example doesn't break across lines
\excnt 2\relax
\begin{farsi}
\ex[everyglb=\englishfont] \begingl
\gla این مثال من است این مثال من است این مثال من است این مثال من است این مثال من است این مثال من است این مثال من است این مثال من است//
\glb in masale man ast in masale man ast in masale man ast in masale man ast in masale man ast in masale man ast in masale man ast //
\glb this example me is this example me is this example me is this example me is this example me is this example me this example me this example me //
\endgl\xe
\end{farsi}
\end{document}
I would recommend sending an email to the Ling-TeX list. John Frampton, the package author, responds to expex
questions there, but he isn't on this site much.