change text for login and messages

Hi there,

First of all, thank you for this plugin. Very helpful. I wanted help with a small text change.

I get the following?prompt when I try to post a private query when I’m not logged in:
You must?login?to view your message.

I want to change above message to:?
You must?login?to send/view your message.

Similarly,

After I sent the Private Query I got the following message:
Message successfully sent.

I want to change the message to :? ?
Your message has been sent successfully.

I tried using the following code, but without success. Any help would be great.

add_filter( 'gettext', function( $translated_text, $text, $domain ){
    if ( 'front-end-pm' == $domain && 'You must login to view your message.' == $text ){
        $translated_text = 'You must?login?to send/view your message.';
    }
    return $translated_text;
}, 20, 3 );