greek text in LaTeX (along with latin text)

It makes sense to define \url to ensure \latintext:

\documentclass{article}

\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[greek]{babel}
\usepackage{hyperref}

\DeclareUrlCommand\url{\def\UrlFont{\latintext\ttfamily}}

\setlength{\textheight}{8cm} % just for this example

\begin{document}

\title{Το θεώρημα του \textlatin{Lamy}\footnote{\protect\url{https://en.wikipedia.org/wiki/Bernard_Lamy}}}
\author{Δημήτριος Σ. Αναγνώστου}
\maketitle

\end{document}

Don't forget \protect when \url is in a footnote.

The most convenient way is to use polyglossia package together with unicode-math package, and to compile your document using xelatex or lualatex engine, respectively. However, you must ensure that you have installed in your system a font that supports greek characters. Here is a MWE:

     \documentclass[11pt]{article}

    \usepackage{unicode-math} % loads fontspec package
    \defaultfontfeatures{Mapping=tex-text}
    \setromanfont[Mapping=tex-text]{GFS Artemisia} % alternative times-font is xits

    \usepackage{polyglossia}
    \setdefaultlanguage{english}
    \setotherlanguage[variant=modern]{greek} 

    \usepackage{hyperref}

    \setlength{\textheight}{8cm} % just for this example

    \title{Το θεώρημα του Lami \footnote{\protect\url{https://en.wikipedia.org/wiki/Bernard_Lamy}}}
    \author{Δημήτριος Σ. Αναγνώστου}

    \begin{document}

 \maketitle

    \end{document}