On StackExchange I found two solutions. The first, which was the
recommended, did not work. The second did work, and is simple:
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[greek,english]{babel}
\usepackage{alphabeta}
\begin{document}
Αυτή η πρόταση ειναι στα ελληνικά, but this one is in english
\end{document}