DUBAI—Russia showed off a prototype of its new fifth-generation warplane at the Dubai Airshow on Sunday as the United Arab Emirates’ deal to buy American F-35 fighter jets makes slow progress. It was the first time the Sukhoi Su-75 Checkmate, unveiled in July, had been shown outside Russia, and according to the Russian state news agency it was visited by an Emirati delegation. A glitzy English-language computer simulation video showed the light tactical warplane, capable of simultaneously carrying five air-to-air missiles, destroying multiple targets at once. The Russian presentation pitched the Checkmate, which was inspected by Vladimir Putin during its July unveiling, as a cost-efficient fighter jet that can fly at speeds of Mach 1.8 and a range of around 1,740-1,800 miles. The Checkmate, due to take its maiden test flight in 2023 and to start production by 2026, has yet to seal an order. The Lockheed Martin F-35 entered …