This article will help you to change it:

https://khanacademy.zendesk.com/hc/en-us/articles/230910428-How-can-I-change-the-language-that-my-mobile-app-uses-

Hi, Ayşe! Great news! With the new Khan Academy v6.4.0 app update, you are now able to complete your Khan Academy exercises in your language of choice, even if it’s different from your device’s main language!

To adjust the language in the app, first, make sure that you have updated your operating system to iOS 13. Then, do the following: click on the profile tab > select the gear icon to access the settings > Select Language & Curriculum > Select the desired language > Done!