\documentclass[a4paper,11pt]{report}
\usepackage{graphicx}


\usepackage{amsmath,amsthm}
\usepackage{amssymb}
\usepackage{amsfonts}
\usepackage{MnSymbol}

%\usepackage{trfrac}


\usepackage[numbers]{natbib}

\usepackage[margin=10mm,font={small}]{caption}

\usepackage{xepersian}
\setfootnoteLR


\settextfont{XB Zar}
\setlatintextfont{Linux Libertine O}
%%\linespread{1.2}

%declare prety version of lambda from txfonts
\DeclareSymbolFont{lettersA}{U}{txmia}{m}{it}
%\SetSymbolFont{lettersA}{bold}{U}{txmia}{bx}{it}
%\DeclareFontSubstitution{U}{txmia}{m}{it}
%\DeclareSymbolFontAlphabet{\mathfrak}{lettersA}
%\let\lambdaup=\undefined
\DeclareMathSymbol{\lambdaup}{\mathord}{lettersA}{21}
\renewcommand{\lambda}{\lambdaup}

\SetMathAlphabet{\mathtt}{normal}{OT1}{cmtt}{m}{n}
\SetMathAlphabet{\mathcal}{normal}{OMS}{cmsy}{m}{n}


\begin{document}
\begin{latin}
\[ \mathtt{this\ is\ a\ test\ with\ typewriter\ font} \]\\
$\mathcal{L}$
\end{latin}
\end{document}