It's likely the Dark Secret wasn't undubbed until now due to the localised versions' game data being packed into the .farc archives... or due to a lack of interest, as the game is not too good no matter how you slice it.
xdelta patch: https://www.dropbox.com/s/2oxqvewu7e0hgcx?dl=1
ROM info:
xdelta patch: https://www.dropbox.com/s/2oxqvewu7e0hgcx?dl=1
ROM info:
Code:
File: Tenchu - Dark Secret (USA).nds SHA1: 173CCEF7A2B65A5B5392E9ECEDBDBA21487D3541 CRC32: F838BBDE