Once completing the installation of Quick Facebook Chat on your store, the wordings in chat box will be English as default.
Facebook can automatically translate the text for the plugin in a local language, to take advantage of automatic social plugin translation, please go to your app dashboard >> scroll down and there's an option called "Language" there >> choose the language you want.
Please refer to this screenshot :
>> Connect with customers & convert them into sales via Facebook Messenger right on your store with Quick Facebook Chat. Get app for FREE.