First off, on the English version of your site, the Login menu item says "Login - Check your Booings" (you're missing a 'k' in Bookings).
As for the redirection, on the English page of your site, the redirection URL encoded into the page is set for /en/book-now . However, when I login using Joomla, it brings me to the /en/ home page instead.
The Spanish redirection is set for /es/reservar . That is bringing me to /en/ though as well. I'm not sure why it's switching me back to English, but it makes it difficult to test. It may also be that the language switch feature is interfering with the redirection.
Can you try:
* Disabling the language switch feature, temporarily, and testing again?
* Check that you don't have any other System or User plugins that may be trying to redirect the user as well. If so, try disabling them.
Let me know how that goes,
Alex