How to write a section in modern greek language?

Question

For the purposes of my thesis I need to write the abstract in greek and the rest of the text in english. I know there are several answers to this question but I am rather confused.

Below you can find my latex manuscript and the section were the greek text should appear:

\documentclass[12pt,journal,onecolumn]{IEEEtran}

\usepackage{cite}
\usepackage[pdftex]{graphicx}
\usepackage{amsmath,amssymb,amsfonts}

%=========================
\let\subparagraph\paragraph
\usepackage{titlesec}

\titleformat{\section} 
  {\normalfont\Large\bfseries}
  {\makebox[40pt][l]{\thesection}}
  {0pt}
  {} 

\titleformat{\subsection} 
  {\normalfont\large\bfseries}
  {\makebox[40pt][l]{\thesubsection}}
  {0pt}
  {}

\titleformat{\subsubsection} 
  {\normalfont\large\bfseries}
  {\makebox[40pt][l]{\thesubsubsection}}
  {0pt}
  {}

\titlespacing{\subsubsection}
  {0pt}
  {1ex plus 0.3ex minus 0.1ex}
  {0.5ex plus 0.1ex minus 0.1ex}
  
\renewcommand{\thesection}{\arabic{section}}
\renewcommand{\thesubsection}{\thesection.\arabic{subsection}}
\renewcommand{\thesubsubsection}{\thesubsection.\arabic{subsubsection}}
%=========================

%-----------------------------
\titlespacing*{\section}
{0pt}{15pt}{15pt}%{5.5ex plus 1ex minus .2ex}{4.3ex plus .2ex}
\titlespacing*{\subsection}
{0pt}{15pt}{15pt}
\titlespacing*{\subsubsection}
{0pt}{15pt}{15pt}
%-----------------------------

\usepackage{parskip}

%\usepackage[english,greek]{babel}

\begin{document}

\title{Bare Demo of IEEEtran.cls\\ for IEEE Journals}
% make the title area
\maketitle

\section*{Abstract in greek language}
This text should be in greek    

\section*{Abstract}
% https://www.andy-roberts.net/latex/contents/
\addcontentsline{toc}{section}{Abstract} % Adding lines to tables of contents
The abstract goes here.

\section*{List of Publications}
% https://www.andy-roberts.net/latex/contents/
\addcontentsline{toc}{section}{List of Publications} % Adding lines to tables of contents

The results of this thesis appeared in the following publications:\\
\underline{\textbf{Journals}}:
\begin{enumerate}
  \item[$1$.] Items are numbered automatically.
  \item[$2$.] The numbers start at 1 with each use of the \texttt{enumerate} environment.
  \item[$3$.] Another entry in the list
\end{enumerate}
\underline{\textbf{Conferences}}:
\begin{enumerate}
  \item[$1$.] Items are numbered automatically.
  \item[$2$.] The numbers start at 1 with each use of the \texttt{enumerate} environment.
  \item[$3$.] Another entry in the list
\end{enumerate}

%--------------
\newpage
\thispagestyle{empty}
\mbox{}
\newpage
%-------------
\tableofcontents
\IEEEpeerreviewmaketitle


\newpage
\section{Introduction}

This demo file is intended to serve as a ``starter file''
for IEEE journal papers produced under \LaTeX\ using
IEEEtran.cls version 1.8b and later

\subsection{Subsection Heading Here}
Subsection text here.

\subsubsection{Subsubsection Heading Here}
Subsubsection text here.

\section{Conclusion}
The conclusion goes here.

\appendices
\section{Proof of the First Zonklar Equation}
Appendix one text goes here.

% you can choose not to have a title for an appendix
% if you want by leaving the argument blank
\section{}
Appendix two text goes here.

%\bibliographystyle{IEEEtran}
%\bibliography{References.bib}

% that's all folks
\end{document}

I tried to use the \usepackage[english,greek]{babel} babel package but the pdfLatex compilation returned an error.

Answer 1

Through trial and error, I just found an answer that seems to do the trick. Instead of importing the babel package as \usepackage[english,greek]{babel}, I imported it as \usepackage[greek,american]{babel}. Then, I wrote the section I needed in greek as

\section*{\foreignlanguage{greek}{Περίληψη}}
\foreignlanguage{greek}{Γεια σου καλημέρα, γεια σου καλησπέρα, γεια σου καλή νότσες!}

Answer 2

You need three actions:

1. make the default language English (you are using Greek as main);
2. use the otherlanguage* environment;
3. substitute the font for Greek, because the standard font used by IEEEtran doesn't support Greek; you can use Tempora instead.

The first action is accomplished with

\usepackage[main=english,greek]{babel}

The third action with

\DeclareFontFamilySubstitution{LGR}{\familydefault}{Tempora-TLF}

See the example below for otherlanguage*

\documentclass[12pt,journal,onecolumn]{IEEEtran}

\usepackage[main=english,greek]{babel}
\usepackage{cite}
\usepackage{graphicx}
\usepackage{amsmath,amssymb,amsfonts}
\usepackage{parskip}

\let\subparagraph\paragraph
\usepackage{titlesec}

%=========================

\titleformat{\section} 
  {\normalfont\Large\bfseries}
  {\makebox[40pt][l]{\thesection}}
  {0pt}
  {} 

\titleformat{\subsection} 
  {\normalfont\large\bfseries}
  {\makebox[40pt][l]{\thesubsection}}
  {0pt}
  {}

\titleformat{\subsubsection} 
  {\normalfont\large\bfseries}
  {\makebox[40pt][l]{\thesubsubsection}}
  {0pt}
  {}

\titlespacing{\subsubsection}
  {0pt}
  {1ex plus 0.3ex minus 0.1ex}
  {0.5ex plus 0.1ex minus 0.1ex}
  
\renewcommand{\thesection}{\arabic{section}}
\renewcommand{\thesubsection}{\thesection.\arabic{subsection}}
\renewcommand{\thesubsubsection}{\thesubsection.\arabic{subsubsection}}
%=========================

%-----------------------------
\titlespacing*{\section}
{0pt}{15pt}{15pt}%{5.5ex plus 1ex minus .2ex}{4.3ex plus .2ex}
\titlespacing*{\subsection}
{0pt}{15pt}{15pt}
\titlespacing*{\subsubsection}
{0pt}{15pt}{15pt}
%-----------------------------


\DeclareFontFamilySubstitution{LGR}{\familydefault}{Tempora-TLF}


\begin{document}

\title{Bare Demo of IEEEtran.cls\\ for IEEE Journals}
% make the title area
\maketitle

\begin{otherlanguage*}{greek}
\section*{Περίληψη}
\addcontentsline{toc}{section}{\foreignlanguage{greek}{Περίληψη}} % Adding lines to tables of contents
Γεια σου καλημέρα, γεια σου καλησπέρα, γεια σου καλή νότσες!
\end{otherlanguage*}

\section*{Abstract}
% https://www.andy-roberts.net/latex/contents/
\addcontentsline{toc}{section}{Abstract} % Adding lines to tables of contents
The abstract goes here.


\section*{List of Publications}
% https://www.andy-roberts.net/latex/contents/
\addcontentsline{toc}{section}{List of Publications} % Adding lines to tables of contents

The results of this thesis appeared in the following publications:\\
\underline{\textbf{Journals}}:
\begin{enumerate}
  \item[$1$.] Items are numbered automatically.
  \item[$2$.] The numbers start at 1 with each use of the \texttt{enumerate} environment.
  \item[$3$.] Another entry in the list
\end{enumerate}
\underline{\textbf{Conferences}}:
\begin{enumerate}
  \item[$1$.] Items are numbered automatically.
  \item[$2$.] The numbers start at 1 with each use of the \texttt{enumerate} environment.
  \item[$3$.] Another entry in the list
\end{enumerate}

%--------------
\newpage
\thispagestyle{empty}
\mbox{}
\newpage
%-------------
\tableofcontents
\IEEEpeerreviewmaketitle


\newpage

\section{Introduction}

This demo file is intended to serve as a ``starter file''
for IEEE journal papers produced under \LaTeX\ using
IEEEtran.cls version 1.8b and later

\subsection{Subsection Heading Here}
Subsection text here.

\subsubsection{Subsubsection Heading Here}
Subsubsection text here.

\section{Conclusion}
The conclusion goes here.

\appendices
\section{Proof of the First Zonklar Equation}
Appendix one text goes here.

% you can choose not to have a title for an appendix
% if you want by leaving the argument blank
\section{}
Appendix two text goes here.

%\bibliographystyle{IEEEtran}
%\bibliography{References.bib}

% that's all folks
\end{document}

The abstract page

The table of contents