Installing a hyphenation file with rules for monotone greek and
english for xelatex and activating a new language monogreeken
in miktex 2.7 for windows vista.

$miktex_dir is "c:\program files\miktex 2.7"

make a copy of file 
$miktex_dir\tex\generic\hyph-utf8\loadhyph\loadhyph-el-monoton.tex
to
$miktex_dir\tex\generic\hyph-utf8\loadhyph\loadhyph-elen.tex

Change the comments, if you want, then change the first message
line to:

\message{Combined UTF-8 hyphenation for monotonic Greek and English}

and then add the following line above "\input hyph-el-monoton.tex":

\input hyphen.tex

Go to start menu, all programs, miktex 2.7, settings. 

In the "Languages" tab, create a new language "monogreeken"
with hyphenation file "loadhyph-elen.tex". Push "Apply".

In the "General" tab, push "refresh FNDB".

In the "Formats" tab, choose "xelatex" and push "Build".

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