We use a dds file, set dimensions for each character. Using special characters, i don't honestly know how they factor in, as i haven't tried any research on them. If you want to start research yourself, the normal characters at least, are contained in data/gfx/gui/font_eng.dds AND font_outline_eng.dds will go with it, same directory.
EDIT:
And a note for editing. There are some symbols and characters i'm not familiar with, though that's not honestly saying much. The default colors in the dds are white, as in all the font is colored white. So you may want to actually invert the image to black for editing, then tell it invert again when you're done, to make it white, before you actually save.
Indeed the normal character used in the game can be found in these files. I found the accents used for the special character I'm looking for in data/gfx/gui/s-jis32bit-1.dds
I guess it's the same file for mabi eu and mabi na (don't have the original pack from these server so I can't check) so it seems that the jpn client doesn't call the proper coordinates to display the desired characters. This is most likely hardcoded as Drahan said and will be difficult to fix (unless this bit of code is in an xml ???).
The only "easy" way would be to replace the code for characters call of this jpn version with the code from na or eu versions but still I dont know where this bit of code actually is.
Anyway thanks for the answers.