Token string is not translatable

The string with the Tokens is not put to the t() function, so the replaced string will be equal for all languages. I suggest wrapping the $submitted variable in submitted_by_node_submitted() in the t() function.

So, change

if ($submitted) {
    return filter_xss_admin(token_replace($submitted, 'node', $node));
  }

To:

if ($submitted) {
    return filter_xss_admin(token_replace(t($submitted), 'node', $node));
  }

See the patch that does exactly this.