Add ability to show all languages

Although it makes sense for locale module to have only 1 language working at a time, I think the idea does not fully apply for i18n module. There are situations where one would like to display nodes of all languages. (Or even set the default to show all languages, if the user haven't chosen any language yet)

For nodes with translation, the default language could then be given more importance and be the one on display, when user choose 'all language'.