> When I say "I'm eating a sandwich", the primary verb is "to eat". I'm telling you my current state as a side effect only.

I'm not sure I understand the uniqueness. In Spanish this would be, in the context of a conversation:

"Qué estás haciendo?"

"Estoy comiendo un sandwich"

Note this is idiomatic and the most common way of saying this, I'm not forcing it.

"Comiendo" (from "comer", "to eat", is the -ing form in Spanish, and it's widely used).