Download and install mkvtoolnix. http://www.videohelp.com/tools/MKVtoolnix
Once installed, look in Programs-->Mkvtoolnix-->MkvmergeGUI. Start MkvmergeGUI. Import your mkv file into mkvmergeGUI and it should show the video stream and both audio streams. Take the check mark out of the box next to the audio stream that you don't want.
Make a name for your new mkv file in the line Output Filename.
Then click on the Start Muxing button at the bottom of the window.
If the mkv file doesn't show two separate audio streams, you are basically out of luck. Someone has mixed the two languages into one stream and it would be very hard to extract the English.