Language learning app Duolingo made its US stock market debut on Wednesday. The company's shares rose as high as 40% on listing, giving it a valuation of $6.5 billion on a fully-diluted basis. The shares opened at $141.4 apiece, up from the initial public offer (IPO) price of $102 apiece. Duolingo raised $521 million by selling about 5.1 million shares.