[Tex/LaTex] keywords in other language

Question

It's the first time I write in Latex and I have faced the following issue:
I have written an abstract in greek and in english, but when I use \begin{keywords}…\end{keywords} in the greek version doesn't show properly: It doesn't translate the word "keywords" in greek.

This is the result I have:

I don't mind having the word in greek; It would also be ok in english as "keywords", but I don't know how to do it.

\documentclass[twoside,a4paper,openright]{book}
\usepackage[english,greek]{babel}
\usepackage[iso-8859-7]{inputenc}
\usepackage[LGR,T1]{fontenc}

\newenvironment{abstract}%
{\cleardoublepage\null \vfill\begin{center} \bfseries \abstractname    \end{center}}%
{\vfill\null}

\newcommand{\keywords}[1]{\par\noindent{\small{\bf Keywords:} #1}} 

\begin{document}

\begin{abstract}
    %Here I write the abstract in greek.
\end{abstract}
\begin{keywords} 
    %keywords in greek as shown in the picture
\end{keywords}

\end{document}

EDIT: Thank you for your answers and I'm sorry for not putting the code earlier.

Answer 1

Here's the idea:

\documentclass[twoside,a4paper,openright]{book}
\usepackage[english,greek]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}

\newenvironment{abstract}
  {%
   \cleardoublepage
   \vspace*{\fill}
   \begin{center}\bfseries\abstractname\end{center}%
  }%
  {\vspace*{\fill}}

\newenvironment{keywords}
  {\par\noindent\small\textbf{\keywordsname:}}
  {\par}

\addto\captionsenglish{\def\keywordsname{Keywords}}
\addto\captionsgreek{\def\keywordsname{Λέξεις-κλειδιά}}

\begin{document}

\begin{abstract}
Αυτό είναι ένα αφηρημένο
\end{abstract}
\begin{keywords}
λέξη ελληνικά \foreignlanguage{english}{Monte Carlo}
\end{keywords}

\begin{otherlanguage}{english}
\begin{abstract}
This is an abstract
\end{abstract}
\begin{keywords}
Word Greek Monte Carlo
\end{keywords}
\end{otherlanguage}

\end{document}

I used UTF-8 and I recommend you do too (change how your text editor saves files, of course; but it should work also with the ISO 8859-7 encoding).

Here's the page with Greek:

and here's the page with English: