I've hit the same problem. It would be great if Content blocks could support language variants like ACS multilingual email deliveries, but they don't (yet). And as for the i18n translation dictionary idea, this applies to classic but unfortunately not to ACS.
So best as far as I know is to include the unsubscribe link directly in an ACS multilingual email delivery and update the link text in each variant. It ain't pretty, but it works. Meantime, feel free to voice your desires for future improved multi-locale support here: Future plans for multilocale campaigns in ACS?