Open the Amazon app on your Android/ IoS phone.
Look at the 'bottom-right' corner of the screen. There is '3 lines menu symbol'. Tap on that.
Scroll down, and tap on the Settings.
Tap on Country & Language.
Now scroll down, and select the country/language you want.
There is another option over there called only 'Language'. That works kind of similar way. But precisely, that has to be used, if you want to select/ change to another language that is used within the same country you have already selected.