Code

documentclass[12pt]{article}
usepackage[margin=1in]{geometry}
usepackage[T1,T2A]{fontenc}
usepackage[utf8]{inputenc}
usepackage[russian, english]{babel}% note - it is recommended to specify the variant of English required to avoid unexpected divergence depending on the version of babel e.g. american or british
usepackage[encapsulated]{CJK}
usepackage{setspace}
doublespacing
usepackage{natbib}
begin{document}
selectlanguage{russian}
слово

begin{CJK}{UTF8}{gbsn}
你好
end{CJK}

selectlanguage{english}
Here is some text in English.

selectlanguage{russian}
слово

end{document}

Explanation

The encapsulated option is required because you are using specialised encodings outside of the CJK environments. For details run

texdoc cjk

Notes

• You should switch languages using either selectlanguage{<language>} (for a general switch until further notice) or foreignlanguage{<language>}{<some text>} (if you just want to typeset a small amount of text in another language).
• You have defined english as the main language. So you need to switch to Russian even if it is the first language used in the document. Passing english is not a great idea because it is ambiguous and has different effects with different versions of babel. Better to specify the variant required such as british or american.

• If Russian should be the main language, you should use something like:

  usepackage[british,russian]{babel}

  The last language passed to babel is assumed to be the first language used and the main language for the document.

A simple way to display Chinese characters is to use the kotex package.

usepackage{kotex}

The good thing about this package is that you don't need to encapsulate your Chinese characters. You can just write things like

你好

and they will display correctly in the PDF.