For how many languages?
Most simple solution is you amend the Header for each menu item, change it to show all the languages, use \r or <linebreak/> to make line break between each language. But that would only really work if you have 2 languages. You can also play around with the column settings and other display parameters to make it look good on screen.
Other method is you create multiple identical menus - one for each language. Once you have the first one setup, you can easily clone it to make a duplicate:
Name each menu as the language you want it to be.
On each menu, update the Header field to show the name in the language for that menu.
Then you can create a Fast Menu on the top of each to enable easy switching between menus, see this tutorial on how to do it:
On another note, have you ensured you are properly locking down the system if you have customer self ordering? You need to make sure customers can only place orders for their own table and also that they have no means to access other tables or anything else on the system. If you have done this already I am interested in how you have accomplished this as it’s something I have been interested in before but not really looked further as I imagine it wouldn’t be straightforward to limit access for each customer.