The Paris-based Relais & Chateaux Association, which represents 377 inns, hotels, resorts and restaurants in 37 countries, has added 20 hotels and 7 restaurants this year, including two Canadian hotels, Langdon Hall, near Toronto, and the Post Hotel at Lake Louise, near Banff.

While six American inns and ranches joined the association last year, including the first New York City property, the Lowell Hotel at 28 East 63d Street, the association added no more American establishments this year."We saw a lot of places that were nice hotels in great settings, but the main failing was the restaurant," said Susan C. Martin, the director of marketing for David B. Mitchell & Co., the American booking agent.

These are the Canadian properties:

Langdon Hall, Cambridge, Ontario N3H 4R8; (519) 740-2100). A country house hotel convenient to Toronto with 40 acres of woodlands and gardens. There are 38 rooms and five suites.

The Post Hotel, 200 Pipestone Road, Lake Louise, Alberta T0L 1E0; (403) 522-3989). In the Canadian Rockies near Banff National Park. There are 83 rooms and 12 suites.

A copy of the 1991 guide is available by sending $5 to: Relais & Chateaux, 2400 Lazy Hollow, Suite 152-D, Houston, Tex. 77063; (713) 783-8033, or to David B. Mitchell & Co., 200 Madison Avenue, New York, N.Y. 10016; (212) 213-2000.