I have not got much in the way of time this second so general explanation is about as good as I can do (ordinarily I might look at the rom). I assume you know what hexadecimal, bytes and bits are.
Japanese text can be a funny beast, as there are several kanji the 256 combinations allowed by an 8 bit encoding scheme is not usually enough for anything other than the barest smatterings of kanji. This means it is 16 bits per character.
However if it is just kana in frequent use then it may well be 8 bit with the menu text graphical
So first, find the text sections, I doubt you know assembler so the "easier" ways will have to suffice:
relative search:
As a kid you undoubtedly made a "code" along the lines of a=1 b=2 c=3....., now the same applies in rom hacking a lot of the time but with hexadecimal (a tbl file is just a list of the numbers used for characters). Game makers are people two (they are lazy) and will often define a character set in the order they appear in the alphabet in question: if you have a section of text in the game like "a bad ace" you can search through the rom for files with a string with an initial character, then one number higher, initial, three up, intial, two up, five up....... Spaces, capitals, punctuation and the slightly rare occurence of a non relative font can frustrate this though.
Transhexition is a good place to start for relative search (in fact for anything above nes I think it is the only really viable solution), it also has the added bonus of being able to readily accept table files.
http://www.romhacking.net/utils/219/
It is not the most full featured editor though so have another on standby (most like x ways forensics or hex workshop).
As mentioned not everything is relative (especially kanji) though so we have to go harder.
Corruption:
A file might read 1565ff7f66ddab which might mean something, by tweaking said file you change what it means. Change a text area and you change the text and you can begin to work out what does what.
Less corruption can lead to you inferring what might be there, it is about as messy as it sounds but it does work.
Try a corrupter or two from
http://www.romhacking.net/ , personally I use my hex editor to invert stuff though (making it unreadable to the program/rom and damn to change back).
A slew of save states for an emulator is needed here to get anywhere fast, also take a look with a tile viewer to save you doing something that is clearly not text.
In further cases of non relative text the font is often the text order: look through the rom in a tile editor and you will likely see a b c d e f.... as graphics.
Add in the possibility of compression (normally LZ compression:
http://gbadev.org/tools.php?showinfo=56 ) and multiple tables (menu, ingame, intro video/chapter start) and you have the reason why rom hacking is considered hard.
Before you ask my preferred tile viewer is tiled 2002 or the original tiled:
http://home.arcor.de/minako.aino/TilEd2002/
If you know chinese (I can not get it to build so we can get an English version) crystaltile2 is fantastic:
http://gbatemp.net/index.php?showtopic=53383
