What you have just described can be trivial tile editing through to full blown ASM hack.
I suppose an example question of a similar nature works here: * is not the first game to avoid using the SDAT sound format, how do I work it from here? There are thousands of ways it could go from here, granted there are few traditional options and the same applies here but hopefully you get where I was heading.
Next we would have to ask what you want out of it; do you want a single byte font for size (kind of pointless on the DS unless you are working in the binaries/overlays), a proper variable width font or something else entirely?
What i'm aiming for is to be able to put more letters into a textbox than the number of japanese hcaracters that can go in. Japanese has kanji which means a whole word in two bytes, which is probably rather hard to translate while preserving the length. Of course I could just try to get more textboxes, but even so, with fat letters that look like there's a space in between each of them is definitely not something you want to have to stare at in a 5 hour+ game.
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General order of play is
Find the game's font and at least divine the format of the font itself. Shining force is a game with a lot of text but note that many puzzle games and games with a low text count will often opt for a bitmap style text setup which places you squarely into graphics hacking territory, menus have also been known to do this.
Some stop here and satisfy themselves with an edited font (hence the half translations of yesteryear not having complete Japanese in the sections that were otherwise untranslated). Things here can range from basic tiles, filtered tiles (fonts are usually two colours and the GBA/DS BIOS have decompression functions for this, see TONC, cowbite and no$gba docs for more on these), split tiles and their cousins of assembled fonts (phoenix wright had some stuff like this for sprites if memory serves, NTFR is somewhat similar to this if memory serves)
so this would be putting the rom through a tile editor and then editing the font to make it skinnier?
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Next up is basic ASM and memory hacking; modern games will often have a somewhat modern font display routine and much like the OAM and regular images (which can also be part of it) you can poke things around and make substantial changes. Note while I say modern values can be hard coded
This is also the stage where my much favoured kludge font width hacks often find themselves (skinny font).
You could get lucky and find a simple 8-16 bit "switch" hack is possible somewhere in here (swap a 16 bit read for an 8 bit read or a similar thing for the in game parser) but it is never wise to count on this.
Finally we have full blown ASM, I am going to link an example project as it covers more than I am willing to type out right now:
http://www.romhacking.net/docs/337/
