\documentclass[11pt]{article}
% okok
\usepackage[utf8]{inputenc}
\usepackage[greek,english]{babel}
%\usepackage{mathptmx}
\usepackage[scaled]{helvet}
\usepackage{alphabeta}

\usepackage{fancyhdr}
%okok.
%okok

\pagestyle{fancy}
\fancyhf{}
%epikefalin
\fancyhead[L]{\nouppercase{\leftmark}}
\fancyhead[C]{epikefalida egrafu}
\fancyhead[R]{\thepage}
\fancyfoot[C]{yposelido egg}
\addto\captionsenglish{
\renewcommand\contentsname{periexomena}
}

\begin{document}
\tableofcontents
\newpage

\section{eisagwgh}
\selectlanguage{english}
hello 

\selectlanguage{greek}
ελλιν \textlatin{hello} περηφανοσ.

paragraph

okok p2

\[ x^2+y^2-\alpha=4\tau +5\alpha \]
% $..$
% $$..$$ \begin{equation}... \end{equation}
\end{document}
