okay now it works – I′ve also contacted the wpml developers.
They mentioned 2 things:
1) The first one is just a notice, the word ‘right’ should be surrounded by quotes.
This is in book-now.php in line 108 and it should look like this:
if($lor == ‘right’) {
2) The second one is replacing the use of _e() with __() in lines 85 and 86, like this:
$myurl = __(get_option(‘rg_book_url’), ‘your_url_booknow’);
$mytext = __(get_option(‘rg_book_text’), ‘your_text_booknow’);
With those 2 fixes the plugin works fine.