An answer to the post right above mine.

Here is how it works :

1) Commands : there are 48 different commands, like "Blue Magic" or "Item". The setup is in the file rdata.cs (_FF9FAbil_ComData). Modifying commands only change the castable abilities in battle ; it doesn't modify which ability whichever character can learn nor his ability list in the menu. If a command gives access to an ability list (like "White Magic" and unlike "Throw"), an external table is used, the "Command Ability list" and the datas of the command are the table position of the 1st ability + the amount of abilities.

2) Command Ability list : by default, it's a table of 192 numbers, which are identifiers of the abilities. The setup is in rdata.cs (_FF9BMenu_ComAbil). Beware that several data ranges are used by (two) different commands : Vivi's Black Magic and Double Black commands use both the same range for instance.

3) Character Command list : there are 20 command pairs, which are the character-specific commands available in battles, when Trance is off. The setup is also in rdata.cs (_FF9BMenu_MenuNormal). As some of the characters have two different command lists through the game (eg. Zidane can have his default commands or his SFX command), there are more command pairs than characters. The link "character - command pair" is made in the game script (external files of type .eb which are stored in the file p0data7.bin) ; here are those links :
- Zidane uses the command pairs 0 (default) and 16 (SFX)
- the other permanent party members use only one command pair, with the same ID as their character ID (1 = Vivi, 2 = Dagger, ...)
- Cinna uses the command pairs 8, 9 (both Steal) and 17 (SFX)
- Marcus uses the command pairs 10, 11 (both Steal) and 18 (SFX)
- Blank uses the command pairs 12, 13 (both Steal) and 19 (SFX)
- Beatrix uses the command pairs 14 and 15 (the same abilities overall but with different commands)

4) Character Trance Command list : same. The setup is in rdata.cs (_FF9BMenu_MenuTrance). There is a 3rd number here, indicating which command changes from normal to trance status (it's only used for AP learning ; the trance abilities of the indicated command are linked to the normal abilities).

5) Character Ability list : it consists of 16 lists, each containing 48 "PA_DATA", which are pairs Ability + AP required. The setup is in FF9/ff9abil.cs (_FF9Abil_PaData). It is the lists of abilities displayed (and usable) in the menu, both for the castable abilities and the supporting skills. It also defines the AP amount required for an ability to be learned. As for the character command lists, the link "character - ability list" is made in the game script and it is the same as the link "character - command pair" so refer to the list above. Note that there are 4 command pairs without abilities (the SFX ones) so nothing can be learned or displayed in the menu while the characters have these command pairs.
Also, another problem (annoying for the OP request) is that the supporting ability sub-menu is always disabled in-game for non-permanent characters. The check for enabling/disabling the supporting ability sub-menu is in FF9/ff9abil.cs at the function FF9Abil_HasAp :
Usually, both those conditions are met or both are not met, together (the only exception without modding is when Zidane uses his SFX commands).

Congrats Albeoris !

There are russian translations already, from what I saw... I'll seek for a link.

EDIT : hum, maybe that's not out yet. People are working on it though.
http://www.4f.ffforever.info/ikonboard.cgi?act=ST;f=2;t=6168;st=735;#entry964340

The original font was custom made. You can find the image with all the letters here :
https://dl.dropboxusercontent.com/u/98687557/Resources/Charmap.png

UnityEx can re-import the text file inside "resources.assets".
As for texts themselves, you can edit them with any text editor (notepad++ for instance).

By the way, I just saw that the formatting in "Localization.txt" is buggy.
Language texts are comma-separated, just like this :

Item,Item,Item,アイテム,Objetos,Objets,Item,Oggetti
Use,Use,Use,つかう,Usar,Utiliser,Benutzen,Usa
Arrange,Arrange,Arrange,せいとん,Ordenar,Ranger,Ordnen,Ordina
KeyItem,Key,Key,だいじなもの,Importantes,Trésors,Wertvolles,Rarità

However, there are several texts with commas inside, which shouldn't be separators but are still handled like ones by the game... They cared about the english texts (there are few commas in them and when there are some, they are wrapped inside quotes so the game read them properly) but the other european languages have bugs because of that.

They are all packed in the file "resources.assets" which is present in the folder
C:\[Steam Folder]\steamapps\common\FINAL FANTASY IX\x64\FF9_Data
Depending on you installation, it may be the one inside x86\FF9_Data instead that is used.

You need an Unity Assets archive browser to access to them (.assets are like .zip, but fewer programs can extract files from them). UnityEx is good for that.

Texts are split between several files in this archive. Namely, you've got to extract those :
Battle Texts "[nb].mes"
Field Dialogs "[nb].mes"
World Texts "[nb].mes"
Field Names "loc_name.mes"
World Places "worldloc.mes"
Spells "aa_name.mes" "aa_help.mes"
Supports "sa_name.mes" "sa_help.mes"
Commands "com_name.mes" "com_help.mes"
Items "itm_name.mes" "itm_help.mes" "itm_btl.mes"
Key Items "imp_name.mes" "imp_skin.mes" "imp_help.mes"
Battle Infos "follow.mes"
Battle Scan "libra.mes"
Battle Spell Naming "cmdtitle.mes"
Choco Menu "ff9choco.mes"
Card Titles "card.mes"
Card Names "minista.mes"
Tetra Master "minigame.mes"
Other Menus "Localization.txt"

Besides "Localization.txt", there are always 7 of them, one for each language. The last has a format a bit different ; all the languages are in the same file.

Texts end with the code [ENDN].

Yes, those are the background files extracted from the PSX version.

Well done ! I'll try that Mono.Cecil API, thanks.
When you say you can replace backgrounds, is that images only or the layout informations as well?