This is currently an option for many languages, including Spanish, Portuguese, French, Turkish, and Polish. 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".
2) On your account settings page, select your primary language, and save the changes.