Duolingo's education app is more popular than ever, but growth is decelerating.If you're confused by Duolingo's (DUOL -6.21%) recent stock market performance, you're not alone. Plenty of investors are shaking their heads after watching its stock price tank by 18% on May 9, the day after it reported record-breaking quarterly earnings.Duolingo's reported results from the first quarter suggest its education application of the same name is gaining popularity at a mind-bending pace. The company even raised its o ...