If you are using one of the current available <a href="http://wordpress.org/extend/plugins/search.php?q=multilingual" target="_blank">multilingual plugins</a>, which permits you writing and publishing posts in several languages, you may also have the need, that native speaking authors should be able to choose their prefered backend language while writing. It's your decision if and how this will be possible. This feature set does not impact your frontend language (defined by config or by any multilingual plugin).