You should link to whatever is the most relevant. If you're posting german content somewhere then it would make sense for the backlink to point to the german version of the website.
Remember user experience - if a user is reading content in German and they click on a link to the author's page, they would expect the language to remain the same.
Don't get too hung up with sending link juice to your root domain. Links to subfolders still benefit the root domain and the value in building language-specific links back to your multilingual pages will be far more worthwhile.