Specs | [UNDUB] Zelda Breath of the Wild (WiiU): Replace dub with Japanese voice acting @SpecsGLA | Uploaded March 2017 | Updated October 2024, 35 minutes ago.
Note: Nintendo have released an official patch that does this but easier and better. Just get that instead.

Get the japanese audio files using JNUSTools:
raw.githubusercontent.com/Maschell/JNUSTool/master/release.zip (Thread about it here: gbatemp.net/threads/jnustool-nusgrabber-and-cdecrypt-combined.413179/)
Make a folder in the same place as JNUS called tmp_00050000101C9300
Get the title.tik for the japanese version of the game (00050000101C9300) and place it in that folder. Get that from wiiu.titlekeys.com/ticket/00050000101c9300.tik
(If you want to get the European voice files and you have the US version then you want tmp_00050000101c9500 and the tik to go with that.)
Make sure to add the common key to the config file: D7B00402659BA2ABD2CB0DB27FA2B656
Open commandline at the folder location (shift+right click) and type (or copy/paste): java -jar JNUSTool.jar 00050000101C9300 -file /content/Voice/.*
That'll get you the japanese voicefiles.

Homebrew apps:
github.com/FIX94/ftpiiu/releases
github.com/aboood40091/mocha/releases/tag/v0.2_fshax
github.com/dimok789/homebrew_launcher/releases
These go in SD:/wiiu/apps/ and then all the .elf files need to live in their own folders.
Go to http://loadiine.ovh/gate.php

Get an FTP client such as:
filezilla-project.org

From there, run mocha_fshax followed by FTPiiuEverywhere and connect via filezilla (or your preferred FTP client) to the IP that FTPiiuEverywhere gave you.
Navigate to storage_usb/title/00050000/101C9*00 with the star as a 5 for EU and a 4 for US (I think). This found in storage_mlc or storage_slc if the game is installed on your internal storage.
Go content/voice/USen and then copy across all the folders from your japanese voice directory (content/voice/JPjp) to the USen directory (or EUes for spanish, EUfr for french, etc). DO NOT copy any .bars files as this may cause issues with some voices in the overworld: http://i.imgur.com/mxePKv5.png
DO NOT delete any files that are on your WiiU, just overwrite as necessary. There are some files necessary to allow the other language versions to run that aren't found on the JP version.

Once it's all copied, open the game. No need for any further homebrewing.
There's a few clips this won't replace. I'm looking into it.

This will work from any language to any language, it will just require you to replace the folders how you like. If you were dedicated, you could mix and match so that Gerudos spoke in Russian and Gorons spoke in German, etc.

This method will not work if you own the disc version of the game. This only works for downloaded versions. Sorry about that. There are ways to do so though.

I highly suggest checking out this thread on undubbing BotW for updates and other methods (such as SD loading): gbatemp.net/threads/tutorial-how-to-undub-zelda-breath-of-the-wild.463642
Many things have changed since this video was created. There may be better methods for you.