Click Download to get the language pack, then run the downloaded pack to install.
This is the Japanese (日本語) language pack and it includes display, help, and proofing tools for the Japanese language.
Important: If you're using a work or school account your IT administrator may have configured your language settings.
After the installation completes, restart Office for your new language settings to take effect. Then return to the Office language settings and configure your display and/or proofing languages.