The Joomla! project does not aggregate the copyrights of its code contributors. No contributor is required to transfer his/her copyright to Joomla! and each individual contribution will forever remain in the ownership of its author. This also applies to translations of the Installation language set as these alone are included in the core release of Joomla!.
Open Source Matters Inc., a not-for-profit organisation, holds the Joomla! project's assets and represents Joomla!'s legal matters. Joomla! project claims copyright over the total aggregated creation which is the CMS and framework. This copyright is held by Open Source Matters Inc. This is why each file in the release is marked "Copyright - Open Source Matters Inc." This does not violate the copyright and ownership of the individual contributions. These always remain the authors' property. Attributions of the copyrights of contributors and other included code elements is detailed in the main copyright file in the root directory of the Joomla! installation. As the contributors do not transfer copyright they instead declare that their code contributions are licensed to Open Source Matters Inc. in a limited and non-exclusive fashion. Limited meaning that Open Source Matters Inc. may only use their contribution if it is released under GNU/GPL for the entire Joomla! CMS (or GNU/LGPL in the specific case of the Joomla! Framework Libraries). Non-exclusive meaning that the contributors are free to do whatever they wish with their own creations including using it for their own releases under any licence they wish (including commercial).