Same as any system.
Find the text, figure out how it works for that game and what you might need to alter/add to get it into the language you want (say adding accents to characters as many will not have them by default), do the alteration.
If you want documentation of the hardware (it does help when hacking games to know what the hardware is like) thenhttp://problemkaputt.de/gbatek.htm
The DS has a file system so is unlike most cartridge based devices that came before it. Said file system means you have names, extensions, sizes, directories and other info you can look at to determine what a file is. It makes certain things a bit harder (tracing for instance is a bit more indirect) and by virtue of it all then anything the devs could cook up they did rather than be as constrained by the hardware as older devices, though it is all usually still pretty nice.
You can pull apart this file system with any number of tools. I like ndstool myself (has frontends in programs called dsbuff and dslazy), nitroexplorer is another, crystaltile2 has some (open the ROM in it, click on the little DS icon on the right hand side of the toolbar, from there is it usually all on the right click or menus. I will note that double click will set the hex window, tile viewer or whatever to the location of that file and the file corresponding to that location is noted down the bottom of the screen), mkds course modifier these days does more than mario kart and has some, tinke is probably the last of the big tools people note in this sort of thing. There are also tools like ndsts that will grab individual files and insert them back in if they are the same size, however those are usually more special requirement to do that.
There are a lot of formats common to things ( http://www.romhacking.net/documents/%5b469%5dnds_formats.htm
) including a text format (usually known for its extension of bmg) but on the text side of things then not much other than Nintendo games will use it. The font format Nintendo provided for devs (NTFR, which has several nice tools to handle it) is used by quite a few games and adding characters to it and altering them is actually pretty easy unlike many older games where you either had to make hard choices, or do a lot of work.
Compression is also commonly used but most people used with the BIOS compressions or things compatible with them.http://members.iinet.net.au/~freeaxs/gbacomp/#BIOS%20Decompression%20Functions
is for the GBA but the DS is much the same.
Tool to handle most thingshttps://www.romhacking.net/utilities/826/
I have a guide focused around GBA and DS stuffhttp://www.romhacking.net/forum/index.php/topic,14708.0.html