Google Market allow me by default to read only comments of my country (Italy). Unfortunately not popular application have only few or even no comments. How can I read comments from other country (USA, UK, etc.)?
When using the Google Market website, go to the bottom of the page and change the language in which you are viewing the Market pages. You will then get user reviews based on the currently selected language.
Note that the system seems to display comments only in the language the users have been using when posting them, so if you are looking at applications that make sense only for a specific country, you will probably see English comments interspersed with others in the native language.
Nothing precludes you from leaving a comment on a Swedish application on the Italian page written in Urdu, should you choose so.
If you search for your app on Cyrket, they show the latest comments from all of the Markets in one list under the app.
AndroLib lets you change which country's Market you're searching by clicking the flags at the top of the page, this also changes the comments displayed to ones from that country's Market.
If you are on your phone, you'll have to change your phone's locale to a more popular one, such as English, in order to see that country's comments.