Thanks Ravi that worked a treat, thank you so much!
I also tried to replace "Sign in with an external account" by creating Account/SignIn/SignInExternalFormHeading.
It didn't seem to work, but when I sign on as Admin, then I can see it changed, but as soon as I close the browser and open it again, the old words are back.
Also, for some reason I'm not able to see the editor anymore. I know what you mean as I've done some changes that way in the past, but we moved our Portal to another instance, and although I'm set up as the administrator I'm unable to see the edit fields when I'm signed in. Is there something I'm missing?
Thanks so much for your help