Google Cloud Translation API provides a simple programmatic interface for translating an arbitrary string into any supported language. Quick translate uses the API for translating missing locales. Google Translate API requires an authentication for pricing. More detailed information here.
Create a text file in project assets and paste your API key into here. (Make sure that the copied key is correct one)
Attach the created key file to Google Authentication File field.
Finally, you can translate missing locales automatically. See here.