This might be tricky.
Most localisations of Japanese games replicate the complete audio setup, and the overwhelming majority of DS games use the SDAT sound format. That typically makes undubbing of a DS game "open Japanese (or whatever version you want) of game, find SDAT, open other region version, insert SDAT, carry on with life". If a game has Japanese that is selectable/able to be activated within the game then a cheat (
http://gbatemp.net/threads/castlevania-portrait-of-ruin-us-undub-cheat.367014/ ) or mod might be made to force that to load but that is rare, similarly Japanese might be buried like unused audio tracks and you can then force that to load. Even if the game does not use SDAT then the solution tends to be the same (overwrite sound file with other file).
Occasionally games will be missing sections (voice work is expensive so they might not do it and just disable it in the other regions) and even other types of audio (I recall one with SDAT and ADX/AHX audio). I am always up for collecting examples of this but it is not common.
Equally some games might use SDAT but in a tricky way like including it in a secondary file system (most games doing such a thing leave the SDAT outside it, a few like Touch Detective do not though).