Several en_GB to en_US.

Also "Show a preview of the transform_grided image". "grided" should be
"gridded", but I also have a problem with the underscore. Should it be
"transform-gridded"? Even so, does it really make sense?
I chose to just read "Show a preview of the transformed image", which I
think is simpler and the most understandable (we don't need to leak the
implementation with a transform grid into the human read text IMO). If
anyone think that was not the right choice, feel free to propose
otherwise.
Thanks to Bruce Cowan for noticing these.