How to write a sentence in Greek script using the MastersDoctoralThesis document class

Question

I am writing my master thesis and I want to write down a sentence in Greek. Some years ago I used in another project for the same situation:

\usepackage[polutonikogreek,english]{babel}

and when I had to write in Greek I was using:

\foreignlanguage{polutonikogreek}{...}

In my current project instead I have this kind of settings:

\documentclass[11pt, english, ...]

What I just tried to do then is adding the polutonikogreek to the document class. Unfortunately it didn't work. By searching online then I found people saying to add the greek.polutoniko and so it became:

\documentclass[11pt, english, greek.polutoniko, ...]

And the package is loaded. Then, in order to write the text in Greek, I should use:

\textgreek{...}

Unfortunately \textgreek{} is a call not recognized. So I moved forward and I saw some answers saying to use:

\usepackage{fontspec}

but it gives me back the error:

The fontspec package requires either XeTeX or LuaTeX.
You must change your typesetting engine to, e.g., "xelatex" or
"lualatex"instead of plain "latex" or "pdflatex".

So, at the end I wasn't able to write my sentence in Greek. Can you help me understanding how to do it please? Thanks in advance!

EDIT: This is my whole document class

\documentclass[11pt, english, singlespacing, headsepline, ]{MastersDoctoralThesis}

Answer 1

For a single sentence in Greek you don't need to load babel-greek. If you are using UTF-8 as the file encoding you can use the first way; otherwise use the transliteration below.

\documentclass{article}
\usepackage[utf8]{inputenc}

\usepackage{textgreek}

\begin{document}

{\textgreekfont Πάτερ ἡμῶν ὁ ἐν τοῖς οὐρανοῖς}

{\textgreekfont\catcode`~=12 P'ater <hm~wn <o >en to~is o>urano~is}

\end{document}

Answer 2

Your custom class is already loading babel at line 155:

So if you load it again in your document, you get an Option Clash error. Now, a good rule of thumb is never to edit classes or packages, so to circumvent this, you can add \PassOptionsToPackage before the \documentclass is declared.

Output

Code

\PassOptionsToPackage{greek.polutoniko, main=english}{babel}
\documentclass[11pt, english, singlespacing, headsepline]{MastersDoctoralThesis}
\usepackage[utf8]{inputenc}

\begin{document}
Cosmos in Greek is \textgreek{Κόσμος}
\end{document}