You have to change the country in the bottom of the Google Flights page, as you always get redirected to the version of the booking website for your chosen country:
(Note that the currency you set in Google Flights influences neither the language of the OTA website nor the currency you are charged in.)
Is there a way to make sure that when you've set Google Flights to English that it will take you to the English language version of the booking site?
No (you can't even make sure that the prices displayed on Google Flights are correct…), but you can increase the likelihood of viewing an English version using some computer (e.g. via VPN or VNC) located in ...