Language ISO code

Where to change?
for site and html area editor...
I need to change charset to -->
iso-8859-5 -->Windows-1251 Cyrillic languages: Azeri, Belarusian, Bulgarian, FYRO Macedonian, Kazakh, Kyrgyz, Mongolian, Russian, Serbian, Tatar, Ukrainian, Uzbek
for site and html area editor...
I need to change charset to -->
iso-8859-5 -->Windows-1251 Cyrillic languages: Azeri, Belarusian, Bulgarian, FYRO Macedonian, Kazakh, Kyrgyz, Mongolian, Russian, Serbian, Tatar, Ukrainian, Uzbek