> usage: htsserver <path-to-html-root-dir> [key value [key
> example: htsserver /usr/share/httrack

The example is bogus - use:
htsserver /usr/share/httrack/

> I checked the existence of lang.def in the path i gave, 
and
> the directory lang is there too, with lots of files in it.

Ensure that you also have the html/ directory in 
/usr/apps/httrack/share/httrack

The /usr/apps/httrack/share/httrack sould look like:

html
icons
lang
lang.def
lang.indexes
templates