To use Russian in ConTeXt MkIV, you need a font that has cyrillic letters, e.g. the DejaVu fonts which are shipped with the ConTeXt Standalone.
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\setupbodyfont[dejavu] \mainlanguage[russian] \starttext Немного русского текста для пробы. \stoptext
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For more details on how to change fonts in ConTeXt, see Characters words and fonts/Fonts in LuaTeX