Tools there are a few general purpose tools (
http://www.romhacking.net/ has them) but most of the time is is standard tables/tile editor (the same format as GBA is used) stuff.
The file system has already been mentioned, it is nice as space issues are not really present (want to extend by 30 megs: chances are you can do it and not have to repoint anything more than the file you are tweaking). See the rom ripping and enhancements link in my signature or look for the .net frontends of DSBuff and DSLazy.
Some links/things to search for:
http://www.gbadat.altervista.org/tools.htm NDSTS is quite nice for simple file finding.
Crystaltile 2, there are some threads around here. It is a general purpose tool with some impressive abilities.
Tahaxan, similar to above although not as advanced in areas and more so in others:
http://tahaxan.arcnor.com/index.php?option...id=39&Itemid=36
Have a read of the threads around this section as they do contain a lot of info that can be of use.
no$gba specs:
http://nocash.emubase.de/gbatek.htm
http://kiwi.ds.googlepages.com/
http://kiwi.ds.googlepages.com/nsbmd.html
http://deufeufeu.free.fr/wiki/index.php?ti...ight_Animations
My own partially finished document (getting a bit outdated though):
http://www.4shared.com/file/3636881/1f5f088d/rom101.html
As for translations: there have been several Chinese translations, Final Fantasy 3 was translated to Thai (kenghot, the hacker/project leader, released the font and script tools I mentioned above), Jump Super stars has one partway finished to English and Italian, there were some smaller ones of to Euro languages (I lost the link though), I was involved in the translation of the US version of New Super Mario Brothers to French (also obsolete) and the arformentioned Jump Ultimate Stars. A few other prjects are also ongoing.