https://www.theverge.com/2018/7/24/17607912/google-docs-grammar-checker-announced

Google Docs is, at long last, getting a grammar-checking feature, which’ll be able to identify mixed up words (like “affect” and “effect”), incorrect tenses, improper uses of commas and clauses, and more. To do all of that, Google says it’ll be relying on machine translation — the same technology it uses to translate between multiple languages. Except, instead of translating a sentence from, say, French to German, it sounds as though it’ll be translating your imperfect writing into a grammatically correct passage.

Details on what the grammar checking feature is capable of and exactly how its AI will work are limited right now. All we really know is that Google is already quite capable when it comes to machine translation — two years ago, the company said its tech was approaching human levels of accuracy. So it makes sense that Google would lean on its already established tech when developing this feature.