That's weird,
I have a site with default language set on Bulgarian and English to switch by ?lang=en.
So it should work with:
i18n::enable();
i18n::set_default_lang('ru');
And when you enter the CMS the default language set to add things will be Russian, when you change it in the CMS to English (for example) and you leave the CMS set on English the website will be in English.. so before you leave let it on Russian again.
Dispite that, with those strings added in your _config.php everything should work (i have it that way and it works) ^^ maybe it's a good idea when you do it to flush the cache ?flush=1
hope you make it!
EDIT:
Well if everything still goes on a mess, you can save your work and start making a new page with the translation mode enabled..this way it should by everything ok ^^
Cheers,