See Language Override, Alternative Menu Item, Template Override, and Layout Override.
Each JA Joomla Template has its own matching 404 page. Below is the screenshot of 404 page...
Retrouvez ici l'activité et les informations de l'association Spirée.
Platea adipiscing justo commodo lobortis montes a vestibulum augue et justo. Eget adipiscing...
Lorem ipsum dolor sit amet consectetuer ut condimentum cursus tincidunt felis. Magna nec dictum...