Looks like the answer is no longer "the answer" since this method is not possible. But I found a solution.
- Add Google account in you phone and make sure that you main calendar appeared in the app.
- Go to you Computer to the next steps.
- Close all browser tabs you have.
- Disable javascript.*
- Change the User Agent to iPad.**
- Make sure above steps works***
- Navigate to Google.com and do you login. (Actually you can do this step first but I did at this point)
- Navigate to https://m.google.com/sync/settings/ . I think this step only works if you browser is set to English.****
- At this point you can see a list with your apps. Find Windows Phone and it will show all your calendars. Check every calendar you want and save. After that sync you phone again.
*In IE9+ go to Settings->Internet Options->Security->Custom Level->Scripting->Active Scripting->Disabled
**IE9: Open developer tools (F12)->Tools->Change user agent string->Custom-> Create a new one with:
Mozilla/5.0 (iPad; CPU OS 5_0 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile/9A334 Safari/7534.48.3
**IE10: Open developer tools (F12)->Tools->Change user agent string->Apple Safari(iPad)
***Go to Google.com and search for something without hit ENTER. If javascript is really disable instant search will not work. Search for "whats my user agent" and you need to see this:
Mozilla/5.0 (iPad; CPU OS 5_0 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile/9A334 Safari/7534.48.3
****In IE9+ go to settings->Internet Options->Language.
Note1: The credit for this steps is this site: http://www.makeuseof.com/tag/sync-multiple-google-calendars-windows/
Note2: With one more step you do the same thing in Windows 8/RT, follow the link.