The following is the list of all the champion skins related by country in the client.

This tab doesn't expose skins related to myths or legends (see here for that)

Countries Name Skins
Argentina
Australia
Brazil
Other than classic MordekaiserSquare Mordekaiser:
Canada
China
Other than classic WukongSquare Wukong and Xin ZhaoSquare Xin Zhao:
Denmark
Other than classic OlafSquare Olaf:
Egypt
Other than classic AmumuSquare Amumu, AzirSquare Azir, NasusSquare Nasus, and RenektonSquare Renekton:
France
Other than classic FioraSquare Fiora:
Germany
Other than classic HeimerdingerSquare Heimerdinger:
India
Ireland
Italy
Japan
Other than classic ShenSquare Shen, AkaliSquare Akali, KennenSquare Kennen, YasuoSquare Yasuo, and ZedSquare Zed:
Korea
Other than classic AhriSquare Ahri:
Lithuania
Mexico
Norway
Other than classic OlafSquare Olaf:
Oceania countries
Poland
Romania
Other than classic VladimirSquare Vladimir:
Russia
Serbia
Other than classic JayceSquare Jayce, ViktorSquare Viktor, and VladimirSquare Vladimir:
Spain
Sweden
Other than classic OlafSquare Olaf:
Thailand
Turkey
United Kingdom
Other than classic CaitlynSquare Caitlyn and CamilleSquare Camille:
United States
Uruguay