https://cooljugator.com/gr/%CE%BC%CE%B9%CE%BB%CE%AC%CF%89 says the third person singular form of μιλάω is μιλά
Ελληνικά means Greek, meaning either the language or Greek things (as it's the neuter form in plural). Greece = Ελλάδα (or Ελλάς, the older form). Ελληνική is the feminine form (sing.) and if you want to use it for the language, you have to use the definite article as well: δε μιλάει την ελληνική (γλώσσα).