Should I disallow drupal path or alias by robots.txt? I have 2 languages enabled on my site English and Japanese.

Japanese is the default and English has prefix "en".

I want search engines to crawl one View page only in Japanese. 

So I decided to use Disallow: /en/my-view/* in robots.txt but my-view has alias.

Should I use Disallow: /en/my-view/* or Disallow: /en/alias-my-view/* ?
You should add both paths, as both of them are accessible.

However, I recommend installing Global Redirect plugin. It will permanently redirect your internal path to the alias (if any). This way you only have to add one path to your robots.txt.