This message is generated by Google upon submission of the form when required fields do not have responses. You can’t find it in the plugin because it isn’t there. It comes through as part of the HTML retrieval from Google just like the form fields do.
If you add a language parameter to the form URL when you set up form within WordPress, Google will translate things like button text, error messages, etc.
Here is a link to an old post on my site which shows how to display the Google generated content in French. This post references the original, and now deprecated, gform shortcode however the solution is the much the same today except instead of appending &hl=fr (fr=French, etc. here is a list of languages although I don’t know if Google Forms support them all), you would append ?hl=fr (or your language parameter of choice) to the end of the URL which ends in /viewform (e.g /viewform?hl=fr).
