** Shares of language learning app Duolingo DUOL.O fall 8.7% to $343 premarket

** Co forecasts 2025 annual core adjusted profit between $259.9 mln and $274 mln, below estimates of $272.1 mln — data compiled by LSEG

** Says forecast is lower than expected because of costs related to AI video call features

** DUOL expects profit margin on sales to be ~3% lower in H1, compared with a year ago

** Co plans to invest heavily in research and development, sales and marketing during H1

** Stock had risen ~15.9% YTD as of Thursday's close, after gaining ~51.3% in 2024

(Reporting by Rishi Kant in Bengaluru)

((Rishi.Kant@thomsonreuters.com))