We build pretty complex systems only based on "natural language" specifications. I think you are conflating specification ambiguity with verification accessibility.

> What does one gain besides familiarity by translation back into a more ambiguous language?

You gain intent verification. Formal languages are precise about implementation, but they are often opaque about intent. A formal specification can be "precisely wrong". E.g. you can write a perfectly precise Event-B spec that says "When the pedestrian button is pressed, the traffic light turns Green for cars"; the formalism is unambiguous, the logic is sound, the proof holds, but the intent is fatally flawed. Translating this back to natural language ("The system ensures that pressing the button turns the car light green") allows a human to instantly spot the error.

> All Martians are green

Modern LLMs are actually excellent at explicating these edge cases during back-translation if prompted correctly. If the formal spec allows vacuous truth, the back-translation agent can be instructed to explicitly flag existential assumptions. E.g. "For every Martian (assuming at least one exists), the color is Green", or "If there are no Martians, this rule is automatically satisfied". You are not translating back to casual speech; you are translating back to structured, explicit natural language that highlights exactly these kinds of edge cases.