[CentOS] Spamassassin under CentOS-6.3

Spamassassin seems to have become more-or-less non-functional
in use with KMail on my server.
It appears to be running OK, but only catches about 20% of my spam.

I wonder if others have experienced this decline?

I suspect that sa-learn is not performing properly.
(I run sa-learn nightly on my spam,
which is saved in ~/Maildir/.Spam/cur/ ,
through the default spamassassin installation.)

Eg I receive more or less identical spam many times
without it being caught.

Also, about 35% of the spam that gets through
is in foreign languates - Russian, Chinese, Arabic and Turkish -
and it would help if I could get rid of this.
I read up about catching such email in spamassassin,
but did not understand exactly what changes are required.

One difficultly I have is that there seem many places
where one can make changes in spamassassin settings.
Am I right in thinking that all such changes can be made
in .spamassassin/user_prefs ?

What should I put in this file to change the score for eg
  3.5 BAYES_99               BODY: Bayes spam probability is 99 to 100%

Any advice or suggestions gratefully received.


-- 
Timothy Murphy  
e-mail: gayleard /at/ eircom.net
tel: +353-86-2336090, +353-1-2842366
s-mail: School of Mathematics, Trinity College Dublin