Air France announced yesterday that it will buy 12 Boeing 737 aircraft for $175 million, a decision that prompted its flight engineers to call a two-day strike for next week, just before Christmas.

The flight engineers oppose the order because the state-run airline intends to operate the two-engine planes with a flight crew of two instead of three, leaving the cockpit with two pilots and no engineer. The Boeing 737s will be the first two-crew airplanes to be used by French airlines.

The union representing most of France's airline pilots has said it won't oppose Air France's purchase of the Boeing 737s and use of a two-person cockpit because this would allow the airline to recover about 30,000 hours of flying time lost when Air France dropped some services because it didn't have the appropriate aircraft.

If the transaction is successful, the strike called by the engineers' union for next Tuesday and Wednesday is considered likely to disrupt flights of Air France and the two other French airlines, Air Inter and UTA. Sources said a legal challenge to the strike from the airlines is possible.

Air France's announcement of the order was made after the Mitterrand government gave it permission to go ahead with the transaction. The government's decision was made by French Secretary of Transportation Charles Fiterman, the Communist Party member regarded as the main defender of unions in the French cabinet, according to French sources here. In a statement on his decision, Fiterman said that because of the competitive and operational environment, the only way to fly those planes is with two crew members.

The dozen 737-200 twin-engine aircraft, costing $14.5 million each, are scheduled for delivery in 1983. They will be powered by Pratt & Whitney engines.

This has been a good week for Boeing, which received new orders for 34 aircraft totaling more than $1.6 billion. On Tuesday, Singapore Airlines said it will buy eight of the latest version of the 747 for more than $1.2 billion. A day later, Boeing announced orders for 14 aircraft totaling about $230 million: eight 727s for Pan American World Airways, four 737s for Nigeria Airways and two 737s for International Lease Finance Corp. of Beverly Hills.

The twin-engine 737s generally have been popular this year; Boeing has received a total of 129 orders for 737s this year compared with only 37 orders for 727s, the company's biggest seller.