DS roms are trivial to expand (you simply add on to the end and rebuild or add on to the end and tweak the filesystem, the hardest part is if the files themselves give themselves a length but that is not that hard either (typically it is the 8th byte onwards, albeit flipped, and maybe a few other ones at the start of the file which are easy enough to spot)), basically space is not an issue.
If you are concerned about pointers they have been figured out (without going back to an earlier post I seem to recall them just being the location offset by the cumilative amount of 00's so far or a simple variant).
Frequently you would have to alter the font (this causes the random garbage you sometimes see in partial translations) but shiftJIS includes Roman characters and they are in the font already (look through the text dump and you will see, song names and level names as well as the odd command).
Basically if a translation appears you merely have to save as a shiftJIS encoded text file, replace the return key with 00's (4 seconds on a hex editor there), generate a pointer map (a bit harder but nothing special) and reinsert into the rom (there are at least 5 tools sitting in front of me not counting my hex editor that can do it)
@Mewgia a good bit of that is the tutorials with which a serious amount of creative license could be taken in translation, machine translation is not a good idea though. If you want a proof of concept (or a partial translation within what my Japanese can afford) I would be happy to oblige.