Path to files are prefixed with 'en' (en/files)

update

update

Also there is something similar, and it happens on some Simpletest tests:

    $this->drupalGet('<front>'); // open default home page
    $this->clickLink('Some button');

which gives the result like:

Valid HTML found on "http://localhost/en"
Clicked link "Some button" (http://localhost/en/en/view/show) from http://localhost/en
GET http://localhost/en/en/view/show returned 404 (14.74 KB).

But this is related to this issue which is fixed by patching:
#944066: Simpletest links in translated site get corrupted

moved into comment

Version 6.x is no longer supported due to Drupal 6 End of Life.