Duolingo, the language-learning app and platform, has closed a $30 million Series F round from CapitalG, Alphabet’s growth equity investment fund. This brings the company’s total funding to $138 million and values the tech startup at $1.5 billion. With 30 million monthly active learners, Duolingo is the most downloaded education app in the world. Since its last funding round in July 2017, the company has grown revenue via its paid subscription offering, Duolingo Plus, and is now also the top-grossing education…