Some more comments (from an English=speaking perspective)
Yes, that is clearly a mistake by a previous translator.
#5. If I use Google to translate “zu Favoriten hinzufügen” it gives me “Add to favorites”.
If I give it “Lesezeichen hinzufügen”, the translation is “Add bookmark”.
So perhaps the bigger question is for the original English word. Should it be “Favorite” or “Bookmark”. I think fashion has changed, and ‘bookmark’ is now more commonly used on the web perhaps? That could be an interesting debate.
In webtrees, are these links to “bookmarks” or are they to “favorite people” ?