Sign in to TickTick on the web.
Click the avatar at the top left corner of the left sidebar > Settings from the menu that appears.
Choose a language in the Language option in the right panel.
P.S. If your language does not appear in the list, and you still wish it can be supported in the app. You can choose to become one of our translation volunteers if you're interested. Click Can't find my own language at the bottom of the list and follow the guide later.