I don't think it can be done, ps2 subtitles work on a totally different level, inputting your own subtitles would require you to have some proper programming skills and professional equipment.
I have seen several examples of this in the market nowadays. GTA San Andreas' and Getaway's cut-scene sub-titles, and complete menus of Wİnning Eleven 8 are translated into a different language.
I don't think that they have been done by completely expert people, I guess there are several relatively methods for this.
I have seen Winning Eleven 8 with all of the menus in Turkish, also GTA San Andreas and Getaway with all sub-titles of the cutscenes in Turkish as well.
I'm sure replacing these sub-titles are possible with some definite methods, I don't think that they are THAT hard do replace.
My killer sig came courtesy of bb "El Jefe" mayo.
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