A free app for Android, by 有限会社シーリス.

MoreLocale 2 is an app that should only be installed on your Android if you are having some issues with its language settings. You can change the language of your device using this tool, but you must restart it. It’s a great app if you bought an Android device outside of your country and don’t fully comprehend the operating system’s language. You only need to install this app, choose a language, and restart your device to get started. This app allows you to select additional locations. If your locale does not appear, you can use the menu’s custom locale function.