Change single Strings in English Version

G’day,
I use for my Blog a special theme, based on twentyeleven. The Problem is, WordPress don’t use an en_EN language-file (I modificated the german one).

I don’t want to show the Author of a post, but if I change the string in the template, the language-file in other languages doesn’t work with the changed string.
So… that was the theory. Here we go with the practice-part:

I see two different solutions. I can change the string in the template and the original phrase in the .po-files. But I have no idea, how I change the original-strings.

But you must know: I had to use different language-files (de_DE, fr_FR, es_ES, it_IT), so it would be easier to include a english language-file with changed phrases. But how can I do this?