I have coded a tool to create automatically language cheat files directly on your DSi. You have to start it only one time and all your roms are in your language. Download and installation can be found here:
http://stormwormsblog.blogspot.com/
I've found a trick to play games in your favorite language on your DSi with Acekard or R4i. Acekard and R4i only work if you set the system language of your DSi to english but then all games without language selection are in english.
Here is the solution how to play these games in your favorite...