I've tried a few fonts, some of these found on internet and some from my PC (e.g. Verdana, Arial, Tahoma, PCSunshoe, Vibrocentric), and i discovered that in spite of the fact that they are .TTF (True-Type) fonts, because of their small filesize they do not contain (i guess) all the symbols and characters needed. So, after i upload them to my phone and set the language to standard english, still they do not display some characters (like R, U, I, and so on). I do not fully understand why, in spite on standard setting of the phone. I must say that i've tried even some romanian characters and still they do not appear correctly.
So, i've reached the following coclusion: the original font of SE K800i (in my case - Sans-Serif Europe and Sans-Serif Europe Bold) must have something extra added compared to the other fonts because it's displayed correctly and beautifully on the screen.

In conclusion, i must find an expensive collection of fonts similar in characteristics to those on my phone, and then i will try again and see what happens.

Anyway, thanks for you help and i believe that what we discussed in here will help others who wants to change the fonts on their phones.

I will keep you posted with my findings.

Keep in touch!