I recommend using Crystal Tile 2. It is the super best, easiest to use application.
You can find it in the utility section here on RHDN: http://www.romhacking.net/utilities/818/
If you use it and mosey over to offset $10B19 and use 1bpp mode, you will see the whole, uncompressed Japanese alphabet (
From your screenshot, it looks like you managed to create a complete table. That was probably hard without knowing the order of the letters? But kudos to you for doing it.
To make an English font, it is as easy as just pasting in a 1bpp bitmap of your font. I literally just type out the alphabet in photoshop, copy it, and paste it directly into Crystal Tile 2, starting wherever the Japanese alphabet started (though it could be anywhere, that is just a convenient place). Be sure to save the ROM afterward.
If the katakana alphabet started with アイウエオ, then in your table, script, and WindHex you can see those letters correspond to values 30, 31, 32, 33, 34 (hypothetically). If you pasted your alphabet starting with "abcde" on top of those letters, now, those values correspond to your English letters. If you think about "7F" not referring to ま, but referring to the tile which stores some graphical data that happens to look like ま, it should be easier to understand. If you put a heart image in that same box and then you run the game, all ま letters would be turned into hearts.
Everyone else on this board is smarter than I am, and I'm sleepy, so I'll leave my explanation as it is. If no one has told you how to deal with pointers by the morning, I'll share what I know on that end.