Here is a list of international cities from where the Air Canada check in from its website or mobile app can be done in order to receive the boarding pass either in email. This can then be shown to airline staff at airport for quick check-in. 

Algiers, Houari Boumedienne, Algeria (ALG) **
Amsterdam, Amsterdam-Schiphol, Netherlands (AMS)
Antigua, V. C. Bird Intl, Antigua and Barbuda (ANU)
Aruba, Reina Beatrix, Aruba (AUA)
Athens, E. Venizelos, Greece (ATH)
Barcelona, Spain (BCN)
Beijing, China (PEK) **
Belize City, Philip S.W. Goldson Int. Airport, Belize (BZE)
Berlin, Berlin-Tegel Int'l, Germany (TXL)
Bermuda, Bermuda (BDA)
Bogota, Eldorado, Colombia (BOG)
Bridgetown, Grantley Adams Int'l, Barbados (BGI)
Brisbane, Australia (BNE) **
Brussels, Luchthaven Brussel, Belgium (BRU)
Budapest, Ferihegy, Hungary (BUD)
Buenos Aires, Ministro Pistarini, Argentina (EZE) **
Cancun, Mexico (CUN)
Caracas, Simon Bolivar Int'l, Venezuela (CCS)
Cartagena, Rafael Nuñez, Colombia (CTG)
Casablanca, Mohammed V Int'l, Morroco (CMN) **
Cayo Coco, Cuba (CCC)
Copenhagen, Denmark (CPH)
Cozumel, Mexico (CZM)
Curaçao, Netherlands Antilles (CUR)
Delhi, Indira Gandhi Int'l, India (DEL)
Dubai, United Arab Emirates (DXB)
Dublin, Ireland (DUB)
Edinburgh, United Kingdom (EDI)
Fort De France, Lamentin Airport, Martinique (FDF)
Frankfurt, Germany (FRA)
Geneva, Geneve-Cointrin, Switzerland (GVA)
George Town, Exuma Int'l, Bahamas (GGT)
Glasgow, United Kingdom (GLA)
Grand Cayman, Owen Roberts Int'l, Cayman Islands (GCM)
Grenada, Point Saline Int'l, Grenada (GND)
Havana, José Martí International Airport, Cuba (HAV)
Holguin, Frank Pais, Cuba (HOG) **
Hong Kong, Hong Kong (HKG)
Huatulco, Mexico (HUX)
Istanbul, Atatürk Int'l Airport, Turkey (IST)
Ixtapa/Zihuatanejo, Mexico (ZIH)
Kingston, Jamaica (KIN)
La Romana, Casa de Campo International, Dominican Republic (LRM)
Liberia, Daniel Oduber Int'l Airport, Costa Rica (LIR) **
Lima, Jorge Chavez Int'l, Peru (LIM)
Lisbon, Portugal (LIS)
London, Gatwick, United Kingdom (LGW)
London, Heathrow, United Kingdom (LHR)
Lyon, France (LYS)
Madrid, Barajas, Spain (MAD)
Manchester, United Kingdom (MAN)
Marseille, Marseille Provence, France (MRS)
Melbourne, Tullamarine, Australia (MEL) **
Mexico City, Benito Juarez, Mexico (MEX)
Milan, Malpensa, Italy (MXP)
Montego Bay, Sangster Int'l, Jamaica (MBJ) **
Mumbai, Chhatrapati Shivaji Int'l, India (BOM)
Munich, Franz Josef Strauss, Germany (MUC)
Nagoya, Chubu Centrair Int'l, Japan (NGO)
Nassau, Bahamas (NAS)
Nice, France (NCE)
Osaka, Kansai Int'l, Japan (KIX)
Panama Tocumen, Panama (PTY)
Paris, Charles De Gaulle, France (CDG)
Pointe-à-Pitre, Pole Caraibes, Guadeloupe (PTP)
Port Au Prince, Mais Gate, Haiti (PAP)
Port of Spain, Piarco Int'l, Trinidad and Tobago (POS)
Prague, Ruzyne, Czech Republic (PRG)
Providenciales, Turks And Caicos Islands (PLS)
Puerto Plata, La Union, Dominican Republic (POP)
Puerto Vallarta, Ordaz, Mexico (PVR)
Punta Cana, Dominican Republic (PUJ)
Reykjavik, Keflavik Int'l Airport, Iceland (KEF)
Rio de Janeiro, Brazil (GIG)
Rome, Leonardo da Vinci Int'l (Fiumicino), Italy (FCO)
Saint Kitts, St. Kitts and Nevis (SKB)
Saint Thomas, Us Virgin Islands (STT)
Samaná El Catey Int'l, Dominican Republic (AZS)
San José, Juan Santamaría Int'l, Costa Rica (SJO) **
San Jose Del Cabo, Los Cabos Int'l, Mexico (SJD)
San Juan, Luis Munoz Marin Int'l, Puerto Rico (SJU)
San Salvador, Bahamas (ZSA)
Santa Clara, Cuba (SNU)
Santiago, Arturo Merino Benitez, Chile (SCL)
Sao Paulo, Guarulhos Int'l, Brazil (GRU)
Seoul, Inchon Int'l, Republic of Korea (South Korea) (ICN)
Shanghai, Pu Dong, China (PVG) **
St Lucia, Hewanorra, Saint Lucia (UVF)
St Maarten, Princess Juliana Int'l, Netherlands Antilles (SXM)
St. Vincent, Argyle Int. Airport, St. Vincent and the Grenadines (SVD) **
Sydney, Kingsford Smith, Australia (SYD) **
Taipei, Taiwan Taoyuan Int'l, Taiwan (TPE)
Tel Aviv, Ben Gurion Int'l, Israel (TLV)
Tokyo, Haneda, Japan (HND)
Tokyo, Narita, Japan (NRT)
Varadero, Juan Gualberto Gomez, Cuba (VRA)
Venice, Marco Polo, Italy (VCE)
Warsaw, F. Chopin, Poland (WAW)
Zurich, Flughafen Zürich-Kloten, Switzerland (ZRH)

It shall be noted that the web check in on Air Canada can be done from all cities of Canada but from the above international cities.