I am using \usepackage{fontspec} together with the text encoding conversion file (ascii-to-serbian.tec) in order to obtain a XeLaTeX->PDF output file in Serbian Cyrillic script. TeX code (see below) is entirely typed in Latin with the Times New Roman output font. However, I am curious is it possible in such a way to produce output PDFs with a Computer Modern font that is the default font in LaTeX. There seems to be an issue in the code when {Times New Roman} is replaced with {Computer Modern}, {Computer Modern Unicode}, or similar. Any help would be highly appreciated.
\documentclass{article}
\usepackage{fontspec}
\setmainfont[Ligatures=TeX]{Times New Roman}
\newfontfamily{\serbianfont}[Mapping=ascii-to-serbian]{Times New Roman}
\usepackage{polyglossia}
\setmainlanguage{english}
\setotherlanguage[Script=Cyrillic]{serbian}
\begin{document}
Novak Djokovi\'{c} in Serbian Latin script
\begin{serbian}
Novak Djokovi\'{c}
\end{serbian}
in Serbian Cyrillic script
\begin{serbian}
\textbf{Novak Djokovi\'{c}}
\end{serbian}
in bold Serbian Cyrillic script
\begin{serbian}
\textit{Novak Djokovi\'{c}}
\end{serbian}
in italic Serbian Cyrillic script
\end{document}