You have a couple options here to accomplish multiple languages within extra pages. Which one you choose depends on how many templates you wish to manage for this feature and/or your ability using smarty code:

Use one template set and place all different translations within a language based smarty if loop. Look to the following support wiki page on language-specific smarty if loop. Using that you would create a smarty if loop with an "if for each language id you have" and place the relevant language specific text within the corresponding section of the if loop. You would place all that within the middle of the "extra body page template" assigned for that extra page. Look to step 3 of this section of the support wiki on how to manage that.

Once you have your multiple languages within the software you create a language specific body template for each extra page. Following the steps within #3 of this section you would assign each language specific body template to it's associated language within that admin tool