For the Windows app, I think it is the Electron app that imposes the spell checking. Not Tana itself. It also drove me crazy and I didn't know how to turn it off. (Something the Tana team should be aware of, in terms of onboarding. I recommend switching it off by default).
Anyway, it is under "Options" in the menu of the app.

How does it work currently, in terms of getting data back after sending the HTTP Request from Tana?
Can something be handed back there, or is the only way that the other platform then sends a call to Tana Input API with the response?