I was correcting an error with $Definition['Activity.JoinCreated.FullHeadline'] and I noticed that that when referring to the user...
%1$s = "You"
%2$s = "You"
%3$s = "you"
%4$s = "your"
%2$s should be "Your" and ideally all of them should only be capitalized at the start of a sentence. I understand conditionally capitalization could be a huge pain, but if I could figure out where the default English text is, I could at least change %2$s.
I'm not a grammar-nut, but the person commissioning the site is an editor, and will definitely be irritated by this.