FARNBOROUGH, England, July 15 (Reuters) - Myanmar-based carrier Air Mandalay has signing a definitive agreement to buy up to 10 Mitsubishi Regional Jets (MRJ), the airline announced on Tuesday.      Based in Yangon, Air Mandalay operates scheduled services to 15 domestic destinations, as well as air charter services for oil companies.     Japan's MRJ seats 70-90 passengers and is powered by Pratt & Whitney's Geared Turbofan engines.        (Editing by Jason Neely)  ((jason.neely@thomsonreuters.com; 0207 542 8825; Reuters Messaging: jason.neely.reuters.com@reuters.net))  Keywords: AIRSHOW BRITAIN/MITSUBISHI AIRMANDALAY