If you can't use command line to manually select the language, you can simply delete the other language files:
In the place you extracted GotR, open up ..\optional_mod_stuff\modified\ and delete all platform*.string files, all static*.strings files, and all voice_script*.le_string files. Then when you build your patch, it only add the default US string files from the original patch archive and you'll only have US language files in-game.