This happens if you change your site’s default language and try editing content that was originally created as a translation. WPML still treats it as a translation, even if it’s now your site’s default language.
When setting up WPML, the language you start in becomes the «original» for all your content – permanently.
For example, if you built your site in English, every page is recorded as an English original. If you later add Spanish and make it your default language, WPML updates the setting – but it doesn’t relabel your existing content.
This means that if you edit a Spanish page, WPML will warn you: «You are about to edit a translation.» Even though Spanish is now your default language, that specific piece of content was created as a translation of English, and WPML is still treating it that way.
What Can I Do?
If you need content in your new default language to be treated as the «original», create a new page for this content in the new default language, and then translate it. This creates new original records in the correct language.