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