\documentclass[mainlanguage=french]{yathesis}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{enumitem}
\makeatletter
\def\YAD@mainlanguage{english}
\expression{meta-abstract}{abstract1}{abstract2}%modification de l'esxpression par défaut
\renewcommand{\YAD@abstract@page}{%
\bgroup%
\enlargethispage{2cm}%
\setlength{\parindent}{0pt}%
\begin{minipage}{\linewidth}
\small%
{%
\bfseries%
\YAD@ifemptyorundef{\printtitle}{%
\YAD@generic@argument@translate{title}%
}{%
\textsc{\printtitle}%
}%
\ifundef{\printsubtitle}{%
}{%
\newline%
\printsubtitle%
}%
\vspace{-.5em}%
\vspace{\z@}%
\begin{center}%
\YAD@abstractname@mainlanguage%
\end{center}%
}%
\par%
\ifboolexpr{%
bool {YAD@abstract@used}%
and not bool {YAD@main@abstract@empty}%
}{%
\usebox{\YAD@abstract@mainlanguage}%
}{%
\YAD@generic@argument@translate{abstract}%
}%