Authors are “mixed” without prejudice or relation to the language in which they write, wrote, or had written. The arrangement is numerical and right now, classic Russian poets tend to dominate. Eventually, there’ll be a separate list for poets who wrote in English, a different list for those who created their work in Russian, and a third one for Francophone poets, but for the time being, we’ve got just one megalist.
Remember that generally, unless these are bilingual literary works or parallel translations, the site won’t show texts in a language other than the one you’ve selected. There is a language switch on top of the page, so you’d have to switch to Russian to read posts in Russian and vice-versa.