Read text defined under __()

You should look at gettext filter hook which is applied to the translated text by the internationalization functions (__(), _e(), etc.). eg:

add_filter( 'gettext', 'my_sanitizer', 20, 3 );
/**
 * my_sanitizer
 * @param  string $translated_text the translation
 * @param  string $text            the origial text from __()
 * @param  string $domain          text domain
 * @return string
 */
function my_sanitizer( $translated_text, $text, $domain ) {

    $translated_text = trim(strip_tags($translated_text)); 

    return $translated_text;
}