Ah, the joy of locales.
There's an obscure setting buried in the Windows locale options that might be your culprit: Language for non-Unicode programs.
Note: changing this setting may require administrative permissions. If your machine is locked down, you may need to talk to your local admin.
The following is how to find this for Windows 10. The setting name hasn't changed in years and years, but Microsoft keeps moving it, so if you're running something earlier, you'll have to find it by some other means.
- Open the Start menu and type Region.
- Open the Region & language settings.
- On the right, click the blue Additional date, time, & regional settings text.
- Open the Start menu and type Control panel.
- Open the control panels.
- Double-click Region.
Once you're looking at the legacy Region settings:
- Click the Administrative tab.
- You should see two options, Welcome screen and new user accounts, and Language for non-Unicode programs. Click the Change system locale button in that second section.
- Select Japanese (Japan) from the dropdown.
By default, Windows systems sold in the US have this set to English (United States), or internal Windows locale decimal number 1033. (See various online lists like this one for the locale codes.) This equates to Excel using ASCII encoding when saving to CSV, which naturally doesn't work too well for high-byte languages like Japanese.
If you change this to Japanese (Japan) or locale number 1041, Excel will export using Shift-JIS, and you'll be able to open your CSV exports in a text editor and see non-mojibaked text.
FWIW, my Win10 locale is set to Japanese (Japan), and when I save an Excel file with Japanese content as CSV (MS-DOS) and open it in Notepad++, I see the encoding on the status bar in the lower right as Shift-JIS, and the Japanese is legible.