The language is automatically determined based on your computer's language settings.
You can set the environment variable MULLVAD_LOCALE to the language you wish to use of the ones we have available:
If using Linux : MULLVAD_LOCALE=en-US ./mullvad-vpn
If using Windows you set it in: System -> Advanced system settings -> Environment Variables -> and then add a System variable called MULLVAD_LOCALE with the value of en-US for English.
If using macOS from a terminal: MULLVAD_LOCALE=en-US /Applications/Mullvad\\ VPN.app/Contents/MacOS/Mullvad\\ VPN
Keep in mind that you will need to restart the Mullvad VPN app for it to take effect.