Russion translation. How I can setup russion language?

There are 2 lang files in folder:
/wp-content/plugins/invisible-captcha/lang/
invisible-captcha-ru_RU.mo
invisible-captcha-ru_RU.po

But message for user and interface of administrator are English.
How I can fix it?
regards,
Mikhail Los

https://wordpress.org/plugins/invisible-captcha/