You would need to override the strings in your language I believe. I haven't tested this but to me it looks like it should work when you replace stuff like
{0.SimFirstName} forgot to have a party for{M0.his}{F0.her} birthday. Or maybe no one wanted to come? Either way, it's very sad.
with
{0.SimFirstName} forgot to have a party for their birthday. Or maybe no one wanted to come? Either way, it's very sad.
The string resource is in Origin Games > The Sims 4 > Data > Strings_ENG_US.package (in a US English install). You can open it with
s4pe (select that resource, hit "Grid", then in the window that opens hit the button with the three dots at the top right) and copy + paste the strings you need into a STBL in a new package that you save in /Mods. The keys need to remain the same so it will override the equivalent strings in your game files. Do
not edit the stuff in Strings_ENG_US.package directly (even when it seems tempting), that'll be overwritten by the next patch.