Thanks for the link above. Just to add a solution –
I found the PHP code three comments above meant that the user did not return to the original page after login. That is really tedious for the user, and will prevent comments. So we had to accept that it had to go to the backend login.
However, with the code you linked to, I could (1) change from the Askimet wording as we wanted, and (2) add a custom registration link, while just keeping the backend login. (I customised the backend.) The code snippet is: