filter_gettext.php

function mycustom_filter_gettext( $translated, $original, $domain ) {
 
    // This is an array of original strings
    // and what they should be replaced with
    $strings = array(
        'Register' => 'Sign up',
        'Confirm and go to payment page' => 'Complete registration',
        // Add some more strings here
    );
 
    // See if the current string is in the $strings array
    // If so, replace it's translation
    if ( isset( $strings[$original] ) ) {
        // This accomplishes the same thing as __()
        // but without running it through the filter again
        $translations = &get_translations_for_domain( $domain );
        $translated = $translations->translate( $strings[$original] );
    }
 
    return $translated;
}
 
add_filter( 'gettext', 'mycustom_filter_gettext', 10, 3 );