If the content is mainly in one language (e.g., English) then use
<html lang="en"> and mark up the Russian bits with
If it's a 50/50 split between the languages, omit the
lang attribute for the
<html> tag and set it on each bit of content instead.
You'll probably want to use UTF-8 as the character encoding, so that you can easily mix Latin and Cyrillic letters on the same page. There are legacy encodings that allows this, too, but UTF-8 is usually the best choice.