I did some research, and I think I have the solution to who "Om Kassoum" is. I believe it's an alternate spelling of "Oum Kalsoum", who was a very popular Arabic singer and poetess from Egypt.

from http://digiboek.50megs.com/oumkalsoum/
"Her name also is written as Umm Kalthum, Umm Kulthum, Oum Kalthum, Um Kalthoum, Umm Kulthume, Oum Kulthum, Oum Kalthoum, Um Kolthoum, Oum Kulthume, Oum Kolthoum, Om Kalsoum, Um Kalsoum, Oom Koolsum, Umm Kalsoum or Omm Kalsoum, but we prefer Oum Kalsoum.

Oum Kalsoum was born at the beginning of this century and died in 1975. She was unquestionably the most gifted singer and musician of this century in the Middle East. She was continuously popular for over 50 years and her songs are still played nightly on any number of Arabic language radio stations. She was the "voice of Egypt". More than twentyfive years after her death, her devoted audience, drawn from all strata of Arab society, still numbers in the millions."

Here's a website where they translate to English one of her love songs called "You Are My Life".
http://www.shira.net/inte-omri.htm
In my humble opinion, the lyrics are quite passionate, and are probably a good indicator of what her other poetry is like. Seems also very Yello-ish to use such powerful lyrics as well, in my opinion. I guess it's a matter of trying to figure out which poem she sang that Yello borrowed, and finally matching them...

As for who "Farida" actually is, it's difficult to browse the web as that name is quite a popular Arabic name. I did find one Arabic singer who goes solely by that name alone. Here's a couple of websites to back that up:
http://www.mondomix.org/womexshowcases2001/artistes/farida.htm (samples here)
http://www.faridamaqam.com/
Here's another Farida, who was a 'superstar' back in the 70s, which makes her a more likely candidate than the other:
http://www.bbc.co.uk/radio3/world/awards2003/profile_kaboul.shtml
http://www.mondomix.org/mix_fr/decouv/festivals/fes2002/jour03/jour03.htm (samples here)

I was able to listen to audio samples of both persons listed above, and neither are singing like the voice on "Le Secret Farida", which means that *neither* are probably the voice. Yello's Farida tends to rasp out the song, not sung with a full-bodied voice. Also, the difficulty in identifying any women who can sing and go by the name "Farida" makes this a pretty daunting task, as there are very many of them. Maybe a certain someone named "Jonas" who is working on a book and has (hopefully) an upcoming interview with a certain someone named "Boris" can ask the man who Farida is, and where they met, etc? ;) Just a thought...

Also, another final thought - someone mentioned how some of the phrases sound French, other lyrics sound Arabic. Perhaps, would this be a pointer to a singer from Morocco? Have either of the Yello gents spent time in Morocco?