babel provides \iflanguage, but its usage for large parts of text is cumbersome.
\documentclass{book}
\usepackage[T2A,T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[russian,english]{babel}
\newif\ifenglish
\newcommand{\chooselanguage}[1]{%
\selectlanguage{#1}%
\iflanguage{english}{\englishtrue}{\englishfalse}}
\begin{document}
\chooselanguage{english}
%\chooselanguage{russian}
\ifenglish
Hello world!
\else
Здраствуй мир!
\fi
\ifenglish
Thank you
\else
Спасибо
\fi
\end{document}
In other words, you write each paragraph (or part of text in general) putting between \ifenglish and \else the English version, and between \else and \fi the Russian text.
You choose the language by commenting or uncommenting the lines after \begin{document}.