I cannot explain why the footer contains so much code but I can explain what is happening inside the div.site-info.
First is the function that executes an action called twentytwelve_credits.
If you were to create a new action and connect it with the twentytwelve_credits hook in either your functions.php file or a plugin then this line of code in the footer would call your action(only when it exists).
The next line is the “Proudly powered by WordPress” with a properly escaped link, escaped title attribute and as you proposed a translated line of text.
These two things that both happen to be in the site-info <div> really aren’t connected to one another and can be used, replaced, or removed without having an affect on one another.
