Where can I find the transition altitude in different countries/regions?
Asia

Hong Kong: 9,000 feet

Africa

South Africa: fixed and to be at least 1000 ft above the highest ground within 25nm radius of an airfield (AIC 20.2)

Europe

Belgium: 4,500 feet (AIP Part 2,, ENR 1.7.2)
Netherlands: 3,000 feet (IFR) / 3,500 feet (VFR) (AIP Part 2, ENR 1.7.1.1)

Oceania

Australia: 10,000 feet MSL (AIP ENR 1.7 (2.1.1))  
New Zealand: 13,000 feet AMSL (CAA CAR 91.425)

North America

Canada: 18,000 feet (SOR/96-433 602.34)
United States: 18,000 feet MSL (14 CFR §91.121)

South America

Argentina: Depends on terminal airspace
Brasil: Depends on terminal airspace
Chile: Depends on terminal airspace
Colombia: 18,000 ft (SAM/IG/12-NE/23)
Ecuador: Depends on terminal airspace
Paraguay: 3000 ft (SAM/IG/12-NE/23)
Perú: Depends on terminal airspace
Uruguay: 3000 ft (SAM/IG/12-NE/23)
Venezuela: Depends on terminal airspace