Heh we usually have to force the other way (developers leave behind English in a game quite often*) or force it for games where it goes with the firmware but people would rather leave it as another for most other games, that said I did port over the force Japanese cheat for the last Advance Wars game for similar reasons (although mine was a bit heavy handed)- http://gbatemp.net/threads/advance-wars-days-of-ruin-dark-conflict-language-mod.254868/#post-4288002
@Chpexo in desmume click view dropdown menu, rotation and pick one of the usual multiples of 90. Vesa mounts are not uncommon either and even if that did not happen quite a few graphics drivers do allow similar rotations these days.
*that reminds me- Magi Nation on the GBA has a full English script buried within it, it is not such a clear cut hack though.
The method varies a fair bit, some games are kind of soft disabled where others have a script but require extensive reworking. The relevant link details the basic idea (the DS firmware has the settings read into general memory similar to how button presses are often handled and the game operates from there- once you have that you can force a value, you can rewrite the handling code or you can do something else entirely like redo the file pointers in memory depending upon how crude or elegant or what is necessary.