Download the locale emulator [xupefei.github.io] and follow the installation instructions closely.

Restart your PC.

Open the game's directory. Normally, it would be
C:\Program Files (x86)\Steam\steamapps\common\ASHA.
The fast way to do this is to click the little cog in the game's page in your library and go from there.


Right click "MW4.exe", hover over "Locale Emulator" and click "Run in Japanese"


In a separate explorer window, open the directory where you extracted the locale emulator. Then. drag and drop "MW4.exe" onto "LEProc.exe".



The game should start in Japanese. if it doesn't, try changing the settings in "LEGUI.exe". But it worked for me without changing anything.

This method is NOT recommended because there's a good chance it'll generate incompatibilities with other programs you might use on your computer.

Open windows settings. On the left bar, click "Time & Language".

1- Under "Preferred languages", click "Add a language" and install 日本語. It'll take a while.
2- Change "Windows display language" to 日本語. You'll be prompted to log off.
3- Change "Country or region" to Japan/日本.

Please note that one or more of these steps might not be necessary. It varies a lot from game to game, but doing all of them is safest way to guarantee the game will boot up in the language you desire.

This game checks the Windows locale in order to determine which display language to use. This is a fine practice... but the game lacks a language setting in the menu, which is a silly practice. That's why you need to jump through hoops to change it.

Changing your system language, region etc surely will.
I'm unaware of locale emulators that let you pick regions other than Japan (many PC games from the country are region locked so this kind of software is in high demand).