This is currently an option for many languages, including Spanish, Portuguese, French, Turkish, Polish, and more. You can see the full list of languages and how much has been translated here.
You will need to be logged in to do this. If you don’t have an account, you can create one here.
1. From any page in Khan Academy (except the Help Center pages), scroll down to the lower banner, and click on "Change language".
This will change the language for your current session.
2. If you want to change the language for all sessions, go to your account settings page (under your name in the top right of the screen), select your primary language, and save the changes.