Download translation for your language and install it through the Joomla installer.
NOTE: If a translation is not 100 percent complete, you may have to translate a few strings. A few strings might not have translated. Please contribute to the translations.
The spirit of opensource lies in the hands of its users. It is a great opportunity for you to contribute to open source. We use the transifex platform to manage translations. It is easy to translate J2store. Just click the contribute now button, register an account with transifex and start translating.
Read this guide for more information