I don't think there's a guide for xenoblade X.
that's not hard if you open the program and look at the available data (file list, extract and pack button)
Extract
just download, extract and launch CriPackedFileMaker.exe.
it's a GUI program, drop your cpk to extract (for example menu.cpk for the menus and titles screen, bdat.cpk for the ingame script, probably other cpk I don't know)
before extracting, check the cpk info (2nd button at the bottom)
CPK Filename:bdat.cpk
File format version:Ver.7, Rev.2
Data alignment:2 048
Content files:7 956
Compressed files:0
Content file size:45 132 476
Compressed file size:45 132 476 (100.00%)
Enable Filename info.:True (251 760 bytes) [Sorted]
Enable ID info.:False
Tool version:CPKMC2.42.00, DLL3.12.00
Select Extract (Top right quadcolor button) and choose a folder.
Modify
go to that folder, rename the "us" folder to "jp"
I don't know if you can delete the additional language folders (you can try)
I don't know if you have to rename common_us to common_jp (you'll have to try without renaming first)
repack
drop the folder into cpk maker, and select "build cpk file..." button at the bottom.
select the option like the original file :
alignement 2048
filemode : filename only (look the original info, there's no ID info)
compression : uncheck (look the original info, it's not compressed : 100%, same size)
replace
rename the generated filename if needed and replace it in your game's content folder.
be sure to keep the original cpk file for all your further extract and tests.
Test and try again
play the game, if it doesn't work, try modfying other files or modifying them differently (renaming filenames too, for example, or keeping/deleting other language folders, etc.)
you'll have to make lot of tests.
You will also need to run the game with Loadiine's TCP log server to see the requested file names.
if you see the game request "content\stream\voice\jp\en" you will have to rename "content\stream\voice\en\us" to match the request. (it shouldn't, it's just an example for logic behind the renaming scheme)