If I change language in W8.1. an update to W10 would change back to original language.
an update is supposed to leave your settings the same. so change to English first then update.
Changing the language in 10 is also easy. as the setting in the notification box all have symbols even if you cannot understand the language 6 clicks will change the language (europepean is easy, have done it from Hebrew, harder because the keyboard is different.)