\usepackage{polyglossia} problem with XeLaTeX

Question

I am a recent user of XeLaTeX and I face a problem with polyglossia package. All the chapters of my thesis are written in english except from the introductory chapter which is in greek. Thus, I use the polyglossia package in the preample

 \usepackage{fontspec}
 \usepackage{xltxtra}
 \usepackage{xgreek}
 \usepackage{polyglossia}
 \setmainlanguage[numerals=arabic]{greek}
 \setotherlanguage[variant=american]{english}
 \newfontfamily\greekfont{FreeSerif}
 \usepackage{newunicodechar}
 \newunicodechar{ʹ}{{\dejavusans ʹ}}

The greek chapter begins with \begin{greek} and ends with \end{greek}. For the english chapters I use \begin{english} and \end{english} respectively.

My main problem is that when I insert a group of figures (e.g. four of them) in the text of an "english" chapter, the letters denoting the figures (a, b, c and d) appear in greek with an upper right tone.

 \begin{figure}[!htb]
  \begin{subfigure}{.5\textwidth}
   \begin{center}
    \includegraphics[width=70mm,scale=0.8]{5_0002_jet1Pt}
    \caption{ }
   \end{center}
  \end{subfigure}
  \begin{subfigure}{.5\textwidth}
   \begin{center}
    \includegraphics[width=70mm,scale=0.8]{5_0003_jet2Pt}
    \caption{ }
   \end{center}  
  \end{subfigure}
 \bigbreak
  \begin{subfigure}{.5\textwidth}
   \begin{center}
    \includegraphics[width=70mm,scale=0.8]{5_0004_muPt}
    \caption{ }
   \end{center}
  \end{subfigure}
  \begin{subfigure}{.5\textwidth}
   \begin{center}
    \includegraphics[width=70mm,scale=0.8]{5_0005_muEta}
    \caption{ }
   \end{center}
  \end{subfigure}
 \bigbreak
 \caption{Transverse momentum (p$_{T}$) of (a) leading jet, (b) subleading jet and (c) muon in truth level in LQ events. Plot (d) shows the pseudorapidity ($\eta$) distribution of muon.}\label{Py_versions_truth_comp}
 \end{figure}

This is the result

As you can see, the letters in the caption appear in english as they should. The problem is located only in the figures. Do you have any idea what 's going on? Is there any possibility that I have chosen wrong letter fonts which do not support greek text? Any suggestion is very very welcome. Thank you very much.

Answer 1

Don't load the xgreek package. It conflicts with other language packages like polyglossia.