Make sure the game is updated to the latest version and that you have the free language pack dlc installed (can be done through the eshop) and then proceed to install the Master Trials dlc with WUP installer. As mentioned before, the game and the dlc have to be installed to the same location and you either need a cfw like mocha or haxchi / coldboothaxchi to make the dlc work. Also, if you have any non-legit dlc installed before applying the free language dlc it might not work. If that is the case, then just head into the Wii U's data management and delete the dlc for now. Then install the language pack from the eshop and after that use WUP installer to re-install the dlc. If everything is correct, you will get a screen that tells you that the dlc has been bought and unlocked the next time you start the game.