Here is the full list of all 99 Flag Collection achievements.
Anarcho-syndicalism
Flag of the Earth
African American Flag
Leather Pride Flag (Black and Blue with Love)
International Bear Brotherhood 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
Rainbow Flag
Agender Pride Flag
Aromantic Pride Flag
Asexual Pride Flag
Olympic Games
North Atlantic Treaty Organization
United Nations
Communist International
Commonwealth of Nations
International Atomic Energy Agency
People's Armed Police
the Imperial Japanese Navy (1889-1945)
People's Liberation Army
Plain Red Banner
Bordered White Banner
Bordered Red Banner
Plain Blue Banner
Bordered Blue Banner
Bordered Yellow Banner
Plain Yellow Banner
Plain White Banner
U.S. Air Force
U.S. Marine Corps
U.S. Coast Guard
U.S. Space Force
U.S. Army
U.S. Navy
Hong Kong
Macau
Lanfang Republic (1777-1884)
Ottoman Empire (1844-1923)
Ryukyu Kingdom (1429-1879)
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)
Republic of West Florida (1810)
California Republic (1846)
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
Argentina
Brazil
Canada
Mexico
the United States
Vatican City
Spain
Romania
Portugal
Italy
Greece
Bulgaria
Ukraine
Russia
Switzerland
Poland
Hungary
the Czech Republic
Germany
Austria
Sweden
Norway
Finland
Denmark
the United Kingdom
the Netherlands
Ireland
Belgium
France
the United Arab Emirates
Turkey
Saudi Arabia
Israel
India
Vietnam
Thailand
Singapore
Malaysia
Indonesia
South Korea
North Korea
Japan
China