Adding conditional text to a PHP Shortcode Template

The strings use in your plugin are using translatable strings ie. _e( 'Inbox', WPC_CLIENT_TEXT_DOMAIN )

It would probably be a better idea to use a translation plugin such as WPML or Polylang as opposed to trying to do this conditionally within the template files.

However, if you prefer to do this within the template file instead, you should maybe look at using the PHP Switch statement (https://www.w3schools.com/php/php_switch.asp), eg.

<?php

$pageid = get_the_ID();

switch ($pageid) {
  case 123:
    echo "Your favorite color is red!";
    break;
  case 456:
    echo "Your favorite color is blue!";
    break;
  case 789:
    echo "Your favorite color is green!";
    break;
  default:
    echo "Your favorite color is neither red, blue, nor green!";
}

?>