To remove foreign language audio from a film so that the original English audio soundtrack is the remaining and therefore default soundtrack, do the following:
1. Download MKVToolNix and install it.
2. Once installed, open the MKVToolNix program folder in the start menu and select mkvmerge GUI. You should see the following window open:
3. Click the add button and select the MKV file you want to edit. Once added it will be visible within the Input tab:
4. Deselect the language you want to remove from the MKV container. Below I am removing French, leaving the English audio file as the default:
5. Once you have deselected the audio file, verify the Output Filename and click Start Muxing:
6. Once the remuxing has completed click OK:
The output will be an MKV file minus the language you deselected earlier. Now you can sit back and enjoy the movie in English!