Compilation crashes

The package french seems outdated, better to use babel, which definetely is a mainline package.

After deleting usepackage{french}it is necessary to delete the *.aux, *.toc, *.log and *.lot files, in short, the auxiliary files LaTeX produced on the way to get a *.pdf. *Beware not to delete the .tex file!!!

I changed your code in some places:

documentclass[a4paper]{report}
usepackage[utf8]{inputenc}
usepackage[T1]{fontenc}
usepackage{graphicx}
usepackage{fullpage}
usepackage{eso-pic}
usepackage{geometry}
usepackage[english, french]{babel}
%usepackage{french}
usepackage[Glenn]{fncychap}
% Font style definition

usepackage{blindtext}

% Interline setting
renewcommand{baselinestretch}{1.5}
newcommand{HRule}{rule{linewidth}{0.5mm}}
newcommand{blap}[1]{vbox to 0pt{#1vss}}
newcommandAtUpperLeftCorner[3]{%
  put(LenToUnit{#1},LenToUnit{dimexprpaperheight-#2}){blap{#3}}%
}
newcommandAtTopCenterPage[2]{%
  put(LenToUnit{.5paperwidth},LenToUnit{dimexprpaperheight-#1}){blap{hbox to 0pt{hss#2hss}}}%
}
newcommandAtUpperRightCorner[3]{%
  put(LenToUnit{dimexprpaperwidth-#1},LenToUnit{dimexprpaperheight-#2}){blap{llap{#3}}}%
}

title{huge{Development and characterization of a gradient structured material elaborated by direct deposition additive manufacturing}}
author{textsc{Corongiu} Chiara}
date{today}

makeatletter
letmytitle@title
letmyauthor@author
letmydate@date
makeatother

%makeatletter
 geometry{hmargin=2cm,vmargin=3cm}
begin{document}
begin{titlepage}
    % AddToShipoutPicture{
    %     AtUpperLeftCorner{1.5cm}{1.8cm}{includegraphics[scale=0.7]{pgarde/uliege.jpg}}
    %    % AtTopCenterPage{1.5cm}{1cm}{includegraphics[width=4cm]{pgarde/mms_logo.png}}
    %     AtUpperRightCorner{1.5cm}{0.6cm}{includegraphics[scale=0.5]{pgarde/Sonaca2.png}}
    % }

    begin{center}
        vspace*{5cm}
        HRule
        vspace{0.3cm}
        textsc{textbf{mytitle}}
        vspace{0.3cm}
        HRule
        vspace*{0.5cm}
        LARGE{textit{Master thesis submitted in partial fulfillment of the requirements
for the degree of Master in Aerospace Engineering by vspace{0.2cm}} myauthor}
    end{center}
vfill
begin{center}
 large{University of Liège - Faculty of Applied Sciences
Academic year 2019-2020}   
end{center}



  %  begin{center}
  %      makebox[textwidth]{includegraphics[width=paperwidth]{data/footer.jpg}}
 %   end{center}

end{titlepage}
ClearShipoutPicture
newpage
pagenumbering{roman}
setcounter{page}{1}
selectlanguage{english}
chapter*{Abstract}
blindtext{}

selectlanguage{french}

blindtext{}

newpage
chapter*{Acknowledgments}
newpage
tableofcontents
listoffigures
listoftables
newpage
pagenumbering{arabic}
setcounter{page}{1}
chapter*{Introduction}

chapter{Powder characterisation}
end{document}

Compile it (twice) an you'll get under ABSTRACT text in english and french.

But if you main question were, how to change the language automagically in all sections, so that the language of chapters and sections would be english, while the text would be french, please edit you question.

By the way: you get the manual of each package with texdoc packagename on the command line (under Windows: cmd.exe). Try texdoc -s packagename.