Aug. 7 (UPI) -- The free language-learning app Duolingo grew by nearly a third in value during Thursday's stock trading after its second-quarter earnings exceeded expectations. Duolingo shares are traded on the NASDAQ stock exchange under the DUOL symbol and reached a high of $465.35 during morning trading but declined to a low of $386.89 in the afternoon. The stock was selling for $397.35 and fluctuating with about an hour left in Thursday's trading, which was a gain of more than $53 and about