This blog post will show you how to change the language to Japanese in SharePoint 2013.
After installing the Japanese language package, run the SharePoint Products Configuration Wizard and click Next.
Once the configuration is complete, click Finish to close the wizard.
To set the Language Settings, go to Site Settings and click on Language Settings in Central Administration.
Check Japanese in Alternate Language(s).
In your Internet Explorer browser, go to Internet Options.
Click the Languages button, then click Add.
Select Japanese (Japan) [ja-JP]) and move the language of your choice to the top using the Move Up button.
Click OK to view in Japanese.