Of the 20% who do speak German, do they also speak English? If not, and 20% of your users don't speak English but instead speak German, then I'd think you've got a pretty large group of non-English visitors. It would make sense then to serve those visitors with T&Cs in their language (English and German). If everybody speaks English, or almost everybody, then probably not a concern.
Are you using a translation service (like Google Translate) on your site? If so, what percent of your traffic translates your site into another language? If any group is sizable (like say, German), I'd offer up T&Cs (and other content!) in those languages along with the English content.
On any content you offer up an alternate language for, be sure to use hreflang: