AIR FARES TO AUSTRALIA are made more liberal to encourage travel to that country during its bicentennial and the World Expo in Brisbane. Qantas Airways established the Australian Bicentennial rate to enhance its promotional fare with more generous conditions of sale and fewer restrictions. The economy-class fare is as low as $995 round trip from the US West Coast to Sydney, through September, after which there will be a seasonal rise. HONEYMOONS GO UPSCALE as American couples plan to spend $2.4 billion on their romantic travel, up 28 percent from four years ago. The estimate comes from a Modern Bride magazine survey. More couples are going abroad, spending an average of $2,785 on their honeymoon. Almost 58 percent of brides surveyed said they would go outside the US, up from 43 percent in 1983. Cruises are also more popular, attracting about 10 percent of those surveyed. Dual incomes are cited as a factor in the increase in luxury travel. FRENCH NATIONAL RAILROADS studies the possibility of putting high-speed double-decker trains in service, to increase capacity by 35 percent. The new cars could be equipped as sleepers. JAPANESE AND EUROPEAN VISITORS TO NEW YORK CITY can enjoy sightseeing trips in their own languages on Gray Line bus tours from now through October. The Japanese tours are available Wednesdays and Saturdays; French and German programs, Wednesdays-Fridays and Sundays; Spanish and Italian tours, Tuesdays, Thursdays, and Saturdays.