ERPNext is an multi-lingual application. It allows each user to select preferred lannguage. Following is how User can customize language in one's account.
On saving User after selecting language, your ERPNext account will be refresh automatically. Then you will see ERPNext translated in your selected language.
Being a System Manager, you can set language in other user's master as well.
Note: For now, we have translation available only for few languages. You can contribute to make translation better, and add new languages from [here](https://translate.erpnext.com).