Looks like the month stuff is not perfect right now. :)
esoTalk will not set any locale information for you, and you have no chance to modify the months with a simple translation file, but the php function strftime() who returns the months, will allways use this locale information.
I have some quick and dirty temporary solutions for you, only that you can test your stuff (this is not a final solution):
1. If you have a own server/vserver, run your apache with another locale information (see: http://forums.freebsd.org/showthread.php?t=11091 )
2. Set the locale inside your esotalk/index.php before the bootstrap gets loaded:
setlocale (LC_ALL, 'ru_RU');
your index.php should look something like this now:
// OMG FIRST COMMENT!!!11!
// Copyright 2013 Toby Zerner, Simon Zerner
// This file is part of esoTalk. Please see the included license file for usage information.
setlocale (LC_ALL, 'ru_RU'); //here you go
and try it again