I searched in joomla forums for a plugin or editing the default configuration file to be able to set the default language when creating an article to all. If you have a multilanguage site, the default language is the language in which you are currently seeing the page, and you can change the language of that article, but manually...
So I decided to create this plugin. You download this directory and you upload it in the backend site and then enable the plugin.
Feel free to change it if you want.