Since you are facing issue in installing optional Japanese language pack, I suggest you to install the language pack by following the below steps and check if it helps.
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Step 1:
Windows 10 has several fundamental and advanced fonts by default, but you can also install any font you desire. Here's a look at how to install them. How to Install the Fonts You Want in Windows 10. How to install fonts that are missing after upgrading to Windows 10.; 3 minutes to read; In this article. Applies to: Windows 10. When you upgrade from the Windows 7, Windows 8, or Windows 8.1 operating system to Windows 10, certain fonts are no longer available by default post-upgrade.
Press Windows + I keys on the keyboard to open Settings menu.
Click on Time and language in settings window.
Click on Region and language from left navigation pane.
Click on Add a language (+ symbol)
Select Japanese language pack.
Now, click on Japanese language pack and click on Set a default button under the Region and language from left navigation pane.
Type Language in the search bar located at the top right corner of Control panel and click on language.
Click on the Japanese language pack on the Language window.
Click on Download and install language pack button under Japanese language pack.
Enter the administrator password if prompted.
Click on Save button.
Refer: Fonts in Windows 10
Refer:Language packs are available for Windows (Applies to Windows 10 Pro).
Download and install additional languages for Windows 10 to view menus, dialog boxes, and other user interface items in your preferred language using the Language Control Panel
Refer: Language packs
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