We use the free Polylang plugin for this. For example, add the Russian version of the site.
Install Polylang from the Wordpress plugin directory. Activate it.
We add a new menu for Russian version
In Oxygen - Templates create the main template "main" (contains the header and footer), "404" template (for 404 pages) and, if necessary, templates for other pages.
In the Polylang settings, check templates.
Add the language version for templates main and 404 (do not edit)
Add the language version for the pages (save, but do not edit). Select template for each page.
Activate Polylang and enjoy :)
WARNING! In the future, before any content editing, be sure to deactivate the Polylang plugin. After editing, activate again. Because Oxygen v.3.3 does not have full Polylang support yet, content can only be replaced in one language when the plugin is active.
More info. If after moving site it switches to language pages with address of the previous site - in the Polyland settings, click Set default language. (Make a backup before)