Meedan - the name means 'town square' - uses machine translation plus a team of 25 journalists and translators.
"Everything that appears on meedan.net is mirrored in Arabic and English – whether it’s the headlines you read, the comments you post, or the articles you share," say its creators.
Users can comment on articles and join cross-language conversations about technology, arts, business and politics.
The San Francisco-based founders say that the system has a translation memory of 3.2 million words, which is growing all the time. Articles are checked over and improved by human translators, and users can ask for help from translators at any time, such as when making comments.
Most of these editors are volunteers, and Meedan says it welcomes new contributors.