I thought that the new site let you book in English through the front page, in a similar way to the UA website, no? I haven't actually made a booking on the new site, but to check availability at least, this is what I do.

1. Login to Asiana Club account
2. Go to the home page
3. Turn the using mileage "switch" on the left side of the page to "ON"
4. On the right side, "Star Alliance" and "Multi-city Flights" will appear. Click on "Star Alliance"
You'll be then taken to the award booking page. Follow through the rest of the process as directed.