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.

(cherry picked from commit f9c170df)