Hello DanielPerez9728524
Good day and thanks for posting in Microsoft community.
We understand when a mobile app like Outlook starts checking spelling in the wrong language, it can really disrupt your workflow. Based on what you’ve already tried, the issue could stem from a deeper setting within the Outlook app or a conflict between iOS settings and the app's configuration.
Even though you deleted the Spanish keyboard from iOS, there might be a lingering preference tied to the mobile app's language settings, or an issue with how Outlook interprets the system-wide language settings.
Please keep in mind there is a dedicated support team for Outlook for Android & iOS issues*.*
Since you have already taken the necessary steps to fix it, I will recommend reaching out to Microsoft's Outlook for iOS experts directly (Get in-app help for Outlook for iOS and Android - Microsoft Support) for more in-depth troubleshooting. They can dive deeper into the backend settings and investigate any configuration conflicts that might be affecting the spell checker functionality on a deeper level.
Hope this helps. Please do not hesitate to contact us if you need further assistance with the issue. We are here to help in any way!
Sincerely,
Bertrand | Microsoft Community Moderator