That only sets the language used in the standard text on the board, ie the instructions, the admin panel pages, the agreement and registration page, the post/reply buttons etc. All the text that the board actually comes with.
It does not and cannot stop your writing a post in another language.
If you had a post in Russian it was probably a spambot and you can stop that with this
http://www.phpbb.com/community/viewtopi ... &t=2122696