Problemer med Engelsk babel

Her er mine dokumentegenskaber.

\documentclass[11pt]{book}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%IMPORT AF PAKKER%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\usepackage{a4}
\usepackage[english]{babel}
\usepackage{t1enc}
\usepackage{theorem}
\usepackage{verbatim}
\usepackage{graphics}
\usepackage{graphicx}
\DeclareGraphicsRule{.JPG}{eps}{*}{`jpeg2ps #1}
\usepackage{epsfig}
\usepackage{fancyhdr}
\usepackage{listings}
\usepackage{latexsym}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


Jeg har ændret \usepackage[danish]{babel} til \usepackage[english]{babel}.
Hvorfor fejler den med beskeden:

! Package babel Error: You haven't defined the language danish yet.

See the babel package documentation for explanation.
Type  H <return>  for immediate help.
...

l.1 \select@language {danish}