In Windows, adding a language, and changing the locale is very dfifferent.
By adding a languge mostly 4 section changes:
Display language (changes the language of menus, settings, and interface)
Keyboard layout (lets you type in that language)
Speech and handwriting recognition (Adding auto correct)
Regional formatting preferences
Changing OS locale:
The system locale affects how non-Unicode programs display text.
It determines how non-Unicode legacy apps (e.g., older software or games) interpret and display text.
It sets the code page used by these programs (e.g., Shift-JIS for Japanese, Windows-1251 for Russian).
It affects some system-wide formats like currency, date, and sorting in certain legacy contexts.
As you see, adding a languate is more of a visual change. And it doesn't change OS behavior.
That's whay your system fails to import a text file (csv) in Japanese. Because you have only added some visual preferences to Office and the engine behind Office (OS) is still the same.
But for us, on this end of the world, who have changed our locale to our language, it doesn't fail.