Single Drupal installation with multiple websites Currently, I have two static HTML folders: 1 for English webiste , and 2 for Thai website. They have the same look and feel but different menu and content. To be precise, the menu and content
in the www.example.org/th are not translated versions of the English site, they are totally different.

URL for the English website is http://www.example.org and http://www.example.org/th for the Thai website.
I want to switch to Drupal 7. So, here's what I have in the to-do list:

Theme:


Create a parent theme (www.exaple.org)
Create a sub theme for (www.example.org/th)


Panel:


I will create a Panel with "th" path for www.example.org/th and then make it use the subtheme (not sure if this is the correct approach)
Create a different menu and will put it in the Panel (not sure if this will work out that way)


Should I create Panel and different menu or Should I go with single Drupal installation that can be used to power multiple websites to get the same result as I currently have with static HTML sites? 
If so, please show me some tips or sample to get started.

Thank you.
Yes, if these two sites are really separate and have nothing in common, I would go with a multi-site setup and a single Drupal installation.

This is supported out of the box, see the Multi-site documentation on drupal.org for more information.