\documentclass{article}
\usepackage{amsmath}
\usepackage[overload]{empheq}
\usepackage{xepersian}         %<<<<<<<<<<<<<<<<<<<<<<<<< Remove and test again.

\begin{document}
	
\begin{alignat}{3}[left =\empheqlbrace]
	& aaaaa &&=bbbbbbbbbbbbbbbbbbbbb  &&=ccc\\ 
	& dddddddddddd &&=eeeeeeeeee &&=f
\end{alignat}
	
\end{document}