My existing user interface and WordPress admin panel language is English.

Only the language of the user interface will change but the language of the WordPress admin panel will be English.

All posts, comments will also use this changes language.

I have added the following code to function.php

function myprefix_live_site_locale( $locale ) {
    if ( ! is_admin() ) {
        return 'ru_RU';
    }
    return $locale;
}
add_filter( 'locale', 'myprefix_live_site_locale' );

it doesn't working.