If anyone else is thinking of trying it you may want to play with this font:
http://www.mediafire.com/?zdddmzd44tj
You'll know why if you try it without it.
And this is how I did it, maybe some unnecesary steps but since it has worked so far I've not tried changing it.
1. Unpack Eu and Jap version.
2. From the EU versions Data\Rom\ folder move over starname.bin, town_thumbnail.bin, wad.bin, misc.bin, slide.bin, face.bin, eye.bin, areatext_*your choice*.bin
3. From EU versions Data\Rom\Text\ move over gd_textres_*your choice*.txrc As well as the 01, 02, 03 files of the same language, and change the language extension to jpn. Ex. gd_textres_jpn.txrc.
4. From EU versions Data\Rom\Scenario\ move over sce_bustup.bin and sce_sbin_*your choice*.bin and rename the last one to sce_sbin_jpn.bin.
That's how I've done it, I expect most of the files in the first step are no different from the jap version and may be left out but since I haven't had any trouble so far I've not gone back to try without some of the files.
I would guess that you might be fine with only areatext_*.bin, gd_textres_*.bin, sce_bustup.bin and sce_sbin_*.bin.
This way everything is in english except the title screen so far (and voices of course
). Good luck.