For frame text, if you're designing before full copy has been written, you can use filler text as placeholder copy to help you progress with your project.
By default, filler text is just a single Lorem Ipsum text field, that autofills on frame resize. Filler text doesn't behave as plain text (i.e., it is not editable, with selection being limited to the entire block) but you can change this to normal editable text per frame or use plain text by default.
If you prefer your own custom text, this can replace the Lorem Ipsum text. For spell checking purposes, you can set the language to go with this text.