Where can I translate the wording for Selling Plan?

If you're using either an App Embed or an App Block to display the selling plans within your theme, you can customize all the wording that's displayed, including translating it into a different language. To do this, head to your Online Store > Theme > Customize:

Then choose Products and search for your membership:

Once you’re on your membership product, find your App Embed or App Block and open it by clicking on it:

Scrolling down in the App Embed or App Block settings, you should now see the option to translate all the wording:

NOTE: If you're looking for how to change the wording of the membership dashboard, you can do that from the Design page within the app: