Here is the full list of all 99 Flag Collection achievements.
China
Japan
North Korea
South Korea
Indonesia
Malaysia
Singapore
Thailand
Vietnam
India
Israel
Saudi Arabia
Turkey
the United Arab Emirates
Belgium
France
Ireland
the Netherlands
the United Kingdom
Denmark
Finland
Norway
Sweden
Austria
the Czech Republic
Germany
Hungary
Poland
Russia
Ukraine
Bulgaria
Greece
Italy
Portugal
Romania
Spain
Vatican City
Canada
Mexico
the United States
Argentina
Brazil
Australia
Chinese Empire under the Qing dynasty (1889-1912)
National Government of the Republic of China (1928-)
Confederate States of America "Blood Stained Banner" (1865)
Republic of West Florida (1810)
California Republic (1846)
Union of Soviet Socialist Republics (1980-1991)
Imperial Banner of the Holy Roman Emperor (1430-1806)
Nazi Germany (1935-1945)
France - Bourbon Restoration (1814-1815)
Ottoman Empire (1844-1923)
Ryukyu Kingdom (1429-1879)
Lanfang Republic (1777-1884)
Hong Kong
Macau
U.S. Army
U.S. Navy
U.S. Air Force
U.S. Marine Corps
U.S. Coast Guard
U.S. Space Force
Bordered Yellow Banner
Plain Yellow Banner
Plain White Banner
Plain Red Banner
Bordered White Banner
Bordered Red Banner
Plain Blue Banner
Bordered Blue Banner
People's Liberation Army
People's Armed Police
the Imperial Japanese Navy (1889-1945)
International Atomic Energy Agency
Commonwealth of Nations
Communist International
North Atlantic Treaty Organization
United Nations
Olympic Games
Rainbow Flag
Agender Pride Flag
Aromantic Pride Flag
Asexual Pride Flag
Bisexual Pride Flag
Genderfluidity Pride Flag
Genderqueer Pride Flag
Intersex Pride Flag
Lesbian Pride Flag
Non-binary Pride Flag
Pansexuality Pride Flag
Polysexuality Pride Flag
Transgender Pride Flag
Leather Pride Flag (Black and Blue with Love)
International Bear Brotherhood Flag
Flag of the Earth
African American Flag
Anarcho-syndicalism
Switzerland