It's as easy as pie.
Download your language and install it through Joomla Installer.
On J2store, we use Transifex platform to manage translations. It involves very simple steps to translate pages on J2store. Just click the 'Contribute now' button, register an account with Transifex and start translating the texts to your target language.
NOTE: A few strings might not translate leaving them slightly less than 100 percent and requiring you to translate just those few string. On completion of it, you are enabling yourself to enjoy the Pro Version for free.
The spirit of open source lies in the hands of its users. It would be great if you keep it alive contributing your time and skills over a rewarding task.
Once completed, please Contact us for free subscription.