sign-in and change it here for your account, applies while signed in to all google sites:
https://myaccount.google.com/language

or don't sign-in, change it for search only.
https://www.google.com/preferences?hl=en&lang=1&prev=https%3A%2F%2Fwww.google.com%2Fpreferences%3Fhl%3Den
this likely uses cookies so it would reset if you clear the one(s) that contain search preferences. the settings link that is actually on google's home page does take you to this.