We've addressed this issue on version 1.2023.22 of the ChatGPT iOS app! You can now select your desired default language by following these steps:
Tap the three dots in the top right corner to access "Settings".
Find and tap the "Main Language" option
Select your desired default language.
How do I check my ChatGPT iOS app version?
To check the version of your ChatGPT iOS app, navigate to "Settings" (accessible via the three-dot icon in the top right corner). You'll see the version information at the bottom of the page.