Using a button for in-progress feedback

An effective way to let users know a process is in progress is to use the button they just clicked.

Here is the initial button state on freetranslation.com:

And here is the button state when clicked, while the translation is in progress:

This approach has two great advantages:  it gives the user feedback right where they gaze is currently fixed (on the button they just clicked)—and, by disabling the button, it prevents users from clicking the button twice (which might happen if they think "nothing" is happening).