Unfortunately, custom unsubscribe text is not an option, mainly due to GDRP law and abuse prevention. The unsubscribe link and text are fixed for each campaign. However, users can change the language of the unsubscribe link by opening Design Settings and finding the Template settings tab on the right side.
By scrolling down a bit you will find Preview and unsubscribe tab. There is an option to change the font, its color and size, or hide the “View in browser” and “Unsubscribe” links.