8. A system comprising one or more computers and one or more storage devices storing instructions that when executed by the one or more computers cause the one or more computers to perform operations comprising: determining that a web page in a source language has been opted in to a translation feature that enables one or more other users to translate the web page to a different target language; obtaining, from the one or more other users, translations of at least a portion of the web page from the source language to the target language; generating a translated web page based on the obtained translations, wherein the translated web page is a translation of the web page into the target language; detecting a request for the web page from a computing device associated with the target language; and outputting, to the computing device, the translated web page.
8. A system comprising one or more computers and one or more storage devices storing instructions that when executed by the one or more computers cause the one or more computers to perform operations comprising: determining that a web page in a source language has been opted in to a translation feature that enables one or more other users to translate the web page to a different target language; obtaining, from the one or more other users, translations of at least a portion of the web page from the source language to the target language; generating a translated web page based on the obtained translations, wherein the translated web page is a translation of the web page into the target language; detecting a request for the web page from a computing device associated with the target language; and outputting, to the computing device, the translated web page. 13. The system of claim 8 , wherein the operations further comprise modifying, in response to determining that the web page has been opted in to the translation feature, a document object model (DOM) of the web page to include JavaScript for providing additional content for the translated web page.