You'd want to freely write in Russian or English. This can be done in
the bilingual environment. There are two options: with and without Babel.
In both cases you must install a ruseng.sty written by V. Volovich
(vvv@vvv.vsu.ru) and found in etc/ in the T2 support macro
distribution. If you do not want to use Babel, just do this:
\usepackage[T2,T1]{fontenc} %% could be OT1 as well but T1 is nicer
\usepackage[koi8-r]{inputenc} %% or whatever input encoding is
\usepackage{ruseng}
If you want to use Babel, go and edit russian.ldf as Vladimir Volovich
suggests. Find
\usepackage[X2]{fontenc} % do not force the switch of \encodingdefault
%\input{x2enc.def}
\def\cyrillicencoding{X2}
and change to
%\usepackage[X2]{fontenc} % do not force the switch of \encodingdefault
\input{x2enc.def}
\def\cyrillicencoding{X2}
Then, you're in business and can do:
\usepackage[russian,english]{babel}
\usepackage[koi8-r]{inputenc} %% Or whaever input encoding is
\usepackage{ruseng}
Babel will yell at you that it could not switch to the Russian hyphenation
patterns just because there are now in the upper part of the character table,
together with English. Ignore it and happy TeXing!
Which one of the two approaches is "better" is hard to say. The first is
a really correct and academic approach which sometimes can be inpractical,
the second could be convinient but it only works for the bi-lingual
environment, and you loose kerning when typsetting Russian. I, myself,
is drifting towards the first one, although I was using the second
approach for a very long time.