\documentclass{book}
% \usepackage[utf8]{inputenc}
% \usepackage[greek, english]{babel}

\usepackage{biblatex}
\addbibresource{sample.bib}


\usepackage{fontspec}  
% Γραμματοσειρά
\setmainfont[Mapping=tex-text]{DejaVu Sans}
\usepackage{xgreek}

\title{My first Latex Project}
\author{Spyros Veronikis}
\date{October 2020}

\begin{document}
\maketitle
\tableofcontents

\chapter{Περί στοιχειοθεσίας}
\section{Introduction}

This is some text.  \\
The quick brown fox jumps over the lazy dog.


Here is some English text (δηλαδή αγγλικό κείμενο) \\

Here is more text. Blah, blah, blah ...

Here comes another section.


\section{A new section}
Hi!
\subsection{This is a subsection}
Subsection text goes here.

\subsection{This is a second subsection}
\textbf{Second} subsection text goes here.\\

Let's cite! The Einstein's journal paper \cite{einstein} and the Dirac's 
book \cite{dirac} are physics related items. 

\printbibliography



\end{document}