Suvayu Ali <fatkasuvayu+linux@gmail.com> writes:

> We could also take this opportunity to provide users an easy way to
> switch between TeX engines.

These two patches goes some of the way towards what you want.
org-latex-pdf-process would also have to patched as well, though.

Test document:

     #+title: test xetex
     #+language: en
     #+options: tex-variant:xetex
     * header
     #+BEGIN_SRC emacs-lisp
       (add-to-list 'org-latex-packages-alist
                    '("AUTO, french" "polyglossia" nil (luatex xetex)))
     #+END_SRC


Result (node the missing inputenc and fontenc):

     \documentclass[11pt]{article}
     \usepackage{graphicx}
     \usepackage{grffile}
     \usepackage{longtable}
     \usepackage{wrapfig}
     \usepackage{rotating}
     \usepackage[normalem]{ulem}
     \usepackage{amsmath}
     \usepackage{textcomp}
     \usepackage{amssymb}
     \usepackage{capt-of}
     \usepackage{hyperref}
     \usepackage{polyglossia}
     \setmainlanguage{english}
     \setotherlanguage{french}

     \author{Rasmus}
     \date{\textit{<2015-07-07 mar>}}
     \title{test xetex}
     \hypersetup{
      pdfauthor={Rasmus},
      pdftitle={test xetex},
      pdfkeywords={},
      pdfsubject={},
      pdfcreator={Emacs 25.0.50.1 (Org mode 8.3beta)}, 
      pdflang={English}}
     \begin{document}

     \maketitle
     \tableofcontents

     \section{add polyglossia}
     \label{sec:orgheadline1}
     \begin{verbatim}
     (add-to-list 'org-latex-packages-alist
                  '("AUTO, french" "polyglossia" nil (luatex xetex)))
     \end{verbatim}
     \end{document}

Rasmus

-- 
However beautiful the theory, you should occasionally look at the evidence