\documentclass{article}
\usepackage{fullpage}
\usepackage{fontspec}
\defaultfontfeatures{Language=Serbian, Style=Alternate, Script=Cyrillic, Mapping=tex-text}
\setmainfont[Path=/home/alessandro/.fonts/liberation-fonts-ttf-1.07.4/, UprightFont=*-Regular, BoldFont= *-Bold, ItalicFont= *-Italic, BoldItalicFont= *-BoldItalic]{LiberationSerif}
\newfontfamily\sans[Path=/home/alessandro/.fonts/liberation-fonts-ttf-1.07.4/, UprightFont=*-Regular, BoldFont= *-Bold, ItalicFont= *-Italic, BoldItalicFont= *-BoldItalic]{LiberationSans}
\newfontfamily\sansnarrow[Path=/home/alessandro/.fonts/liberation-fonts-ttf-1.07.4/, UprightFont=*-Regular, BoldFont= *-Bold, ItalicFont= *-Italic, BoldItalicFont= *-BoldItalic]{LiberationSansNarrow}
\newfontfamily\mono[Path=/home/alessandro/.fonts/liberation-fonts-ttf-1.07.4/, UprightFont=*-Regular, BoldFont= *-Bold, ItalicFont= *-Italic, BoldItalicFont= *-BoldItalic]{LiberationMono}
\usepackage{xunicode}
\usepackage{xltxtra}
\usepackage{nopageno}
\usepackage{color}

\begin{document}
\fontsize{18}{24}\selectfont
Liberation Serif Regular: б

\textbf{Liberation Serif Bold: б}

\emph{Liberation Serif Italic: б г д п т ш}

\emph{\textbf{Liberation Serif Bold Italic: б \textcolor{red}{г} д \textcolor{red}{п} т ш}}

\

\fontsize{18}{24}
\sans{Liberation Sans Regular: б}

\sans{\textbf{Liberation Sans Bold: б}}

\sans{\emph{Liberation Sans Italic: б \textcolor{red}{г д} п т ш}

\sans{\textbf{\emph{Liberations Serif Bold Italic: б \textcolor{red}{г д} п т ш}}

\

\fontsize{18}{24}
\sansnarrow{Liberation Sans Narrow Regular: \textcolor{red}{б}}

\sansnarrow{\textbf{Liberation Sans Narrow Bold: \textcolor{red}{б}}}

\sansnarrow{\emph{Liberation Sans Narrow Italic: б \textcolor{red}{г д п} т \textcolor{red}{ш}}}

\sansnarrow{\emph{\textbf{Liberation Sans Narrow Bold Italic: б \textcolor{red}{г д п} т \textcolor{red}{ш}}}

\

\fontsize{16}{20}
\mono{Liberation Mono Regular: б}

\mono{\textbf{Liberation Mono Bold: б}}

\mono{\emph{Liberation Mono Italic: б г д п т ш}}

\mono{\emph{\textbf{Liberation Mono Bold Italic: б г д п т \textcolor{red}{ш}}}}

\end{document}