Here I would like to know, how to translate admin part as well.
How a person who do not know english, can use adminstration interface which is in english. Please clarify this point.
Moreover I have created bi-lingual site with two languages: English and Portuguese. English pages are coming alright from the url sitename/en, but when I am using sitename/pt, I can see the page in portuguese saying access denied in portuguese.
From what you wrote, it is right that you got a portuguese error message from portuguese siteaccess
About translating the admin interface , it depends on what do you want to achieve, AFAIK (until 3.4 version) there is not a good solution for this, a workaround is adding an extra siteacces based on admin , following the same procedure used for user siteaccess , this should work with the drawback that you should create every object using the main language admin, once created then you can use the translated admin to edit the objects or admin the site