The app's language and your planner's language are separate

There are two language settings, and they do different jobs.

The app's language is yours

It sets the menus, buttons, messages and dates you see. You change it in your Profile, under Account Settings, then Language. It follows you on every device, and each collaborator sees the app in their own language.

The planner's language belongs to the planner

It sets the words printed on that planner's pages, the tabs along its top, and its PDFs and printouts. You choose it when you set the planner up (Planner language, which is English unless you pick another), and the planner's owner can change it later in Planner settings.

Everyone who opens a shared planner sees its pages in the planner's language, whatever language their app is in, so a shared planner reads the same for everyone.