PDF Viewer by default uses the system language set on your device. If your system language is set to English then PDF Viewer will also be translated in English.
Before iOS 13
You can change your system language by going into the Settings.app > General > Language & Region > iPhone/iPad Language. Set your preferred language, let the device make the changes and enjoy PDF Viewer in the language you want.
iOS 13 and later
Since iOS 13 you can set the preferred language for each app separately. To do this go into the Settings.app > Scroll down to PDF Viewer > Set preferred language