Yes, it is safe to use Duolingo — it will not infect your device or steal your data. Duolingo provides a legitimate service and has, so far, not suffered any data breaches. However, while Duolingo safety is not in question, your own security while you use Duolingo depends on your circumstances. For example, if you connect to an unsecured Wi-Fi hotspot to learn languages over lunch, Duolingo might be safe to use — but browsing the web could put you at the mercy of any hackers lurking on the network.