If you really need to use that buggy class, you have to edit it.
Make a copy, call it myCUEDthesis.cls and change the first lines, that originally reads (with line numbers added for clarity)
1 %
2 % CUEDthesis v1.1
3 % By Harish Bhanderi <harish.bhanderi@cantab.net
4 % Version 1.0 released 15/07/2002
5 % Version 1.1 released 14/07/2010
6 %-------------------------- identification ---------------------
7 %\NeedsTeXFormat{LaTeX2e}
8 \ProvidesClass{CUEDthesisPSnPDF}[2010/07/14 v1.1 CUED thesis class]
9 %-------------------------- initial code -----------------------
10 \newif\ifCU@bookmode
11 \CU@bookmodetrue
12
13 \DeclareOption{report}{\CU@bookmodefalse}
14 \DeclareOption{book}{\CU@bookmodetrue}
15 \ProcessOptions\relax%
16
17 \ifCU@bookmode
18 \DeclareOption*{\PassOptionsToClass{\CurrentOption}{book}}%
19 \ProcessOptions\relax%
20 \ifx\pdfoutput\undefined%
21 \LoadClass[dvips, a4paper]{book}%
22 \else%
23 \LoadClass[pdftex, a4paper]{book}%
24 \fi%
25 \else
26 \DeclareOption*{\PassOptionsToClass{\CurrentOption}{report}}%
27 \ProcessOptions\relax%
28 \ifx\pdfoutput\undefined%
29 \LoadClass[dvips, a4paper]{report}%
30 \else%
31 \LoadClass[pdftex, a4paper]{report}%
32 \fi%
33 %\renewcommand{\refname}{References}%
34 \fi
35
36 %\DeclareOption{book}{%
37 %}%
38 \usepackage{setspace}
39 \usepackage{tocbibind}
40 \usepackage{amssymb}
41 %\usepackage{graphicx}
42 \usepackage{fancyhdr}
43 \usepackage{eucal}
44 \usepackage[english]{babel}
45 \usepackage[usenames, dvipsnames]{color}
46 \usepackage[perpage]{footmisc}
47 \usepackage[square, sort, numbers, authoryear]{natbib}
48 \usepackage{ifthen}
49 \usepackage{ifpdf}
to the following
% CUEDthesis v1.1 with modifications by Isai
%%%%%%%
% By Harish Bhanderi <harish.bhanderi@cantab.net
% Version 1.0 released 15/07/2002
% Version 1.1 released 14/07/2010
%-------------------------- identification ---------------------
%\NeedsTeXFormat{LaTeX2e}
\ProvidesClass{myCUEDthesis}[2013/01/26 v1.1m CUED thesis class]
%-------------------------- initial code -----------------------
\newif\ifCU@bookmode
\CU@bookmodetrue
\DeclareOption{report}{\CU@bookmodefalse}
\DeclareOption{book}{\CU@bookmodetrue}
\ProcessOptions\relax%
\ifCU@bookmode
\DeclareOption*{\PassOptionsToClass{\CurrentOption}{book}}
\ProcessOptions\relax
\LoadClass[a4paper]{book}
\else
\DeclareOption*{\PassOptionsToClass{\CurrentOption}{report}}
\ProcessOptions\relax
\LoadClass[dvips, a4paper]{report}
\fi
\usepackage{setspace}
\usepackage{tocbibind}
\usepackage{amssymb}
\usepackage{fancyhdr}
\usepackage{eucal}
\usepackage[usenames, dvipsnames]{color}
\usepackage[perpage]{footmisc}
\usepackage[square, sort, numbers, authoryear]{natbib}
\usepackage{ifthen}
\usepackage{ifpdf}
Then you'll be able to say in your document
\documentclass[twoside,11pt]{Classes/myCUEDthesis}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc} %%% use the actual input encoding
\usepackage[spanish]{babel}
One might also use the original class and say
\documentclass[twoside,11pt,spanish]{Classes/CUEDthesisPSnPDF}
starting the document with
\begin{document}
\selectlanguage{spanish}
However it not recommendable, in my opinion, to load babel in a class file, because it's a typical user setting. Moreover, the part I've modified is really buggy in the original, because it will never select DVI output, but always setup the document for PDF output.
Every construction like
\ifx\pdfoutput\undefined
is wrong, because \pdfoutput is defined in all cases (at least if the TeX distribution is less than five years old, approximately).
\usepackage[spanish]{babel}? – Kurt Jan 26 at 18:24classicthesisis compatible with that class? It can work only with classes based onscrbookorscreprtand I don't think it's good with your custom class. – egreg Jan 26 at 18:49