% Document MUST be built with XeLaTeX!
\documentclass[12pt,a4paper]{article}

% Thanks to https://tex.stackexchange.com/a/136926
\usepackage{fontspec}
\defaultfontfeatures{Ligatures=TeX}
\setmainfont{CMU Serif}
\setsansfont{CMU Sans Serif}

\usepackage[bulgarian,english]{babel}

\title{Документ}
\author{Joe}
\date{2021}

\begin{document}

\maketitle

\end{document}