language name (used only for display)
// slug -> language code (ideally 2-letters ISO 639-1 language code but I currently use it only as slug so it doesn't matter)
// description -> WordPress locale for the language. Here if something wrong is used, the .mo files will not be loaded...
// term_group -> order
// adds noheader=true in the action url to allow using wp_redirect when processing the form ?>