Installing a hyphenation file with rules for monotone greek and
english for xelatex and activating a new language monogreeken
in texlive 2007 and ubuntu/debian.

My system has texlive-xetex and texlive-lang-greek installed.

sudo vim /usr/share/texmf-texlive/tex/generic/xu-hyphen/utf8elen.tex

and type the following in utf8elen.tex:

\begingroup
\message{UTF-8 hyphenation patterns for Modern Greek and English}%
\expandafter\ifx\csname XeTeXrevision\endcsname\relax
  \input hyphen
\else
  \input hyphen
  \input utf8-grmhyph4.tex
\fi
\endgroup
\endinput

sudo vim /etc/texmf/language.d/10texlive-lang-greek.cnf
and add the following line at the end of 10texlive-lang-greek.cnf:
monogreeken	utf8elen.tex

sudo texhash

sudo update-texmf

sudo update-language

sudo fmtutil-sys --byfmt xelatex

Now, you can use \setlanguage{monogreeken} in your tex files.
This language changes only the hyphenation rules.