See JOOMFISH 1.7 Tips & tricks in the Joomla! Forums.

Taken from that post:

TIP 1 - HOW TO LOAD A CSS DEPENDING OF THE LANGUAGE FRONT-END SWITCH ?

1. Add the following tag in your template index.php file:

<link href="<?php echo $mosConfig_live_site;?>/templates/
YOUR TEMPLATE NAME/css/<?php echo $mosConfig_lang;?>.css" rel="stylesheet" type="text/css"/> 

This will write CSS files, depending of the selected language: english.css, spanish.css, german.css, italian.css, portuguese.css, etc...

2. If your defaut joomla language is set to english, then leave english.css empty & place it in the /templates/YOUR TEMPLATE NAME/css/ directory. Customize the other css files according to your needs. The most natural use is to change the path to an image file:

#whatever { 
	background-image: url('/"../images/header_german.gif"');
	for the german.css file 
	}   
#whatever { 
	background-image: url('/"../images/header_spanish.gif"'); 
	for the spanish.css file 
	} 

Place the produced CSS files into the /templates/YOUR TEMPLATE NAME/css/ directory

How to select a template based on the language front-end switch?

The most easy solution is to add the following lines in your index.php

  // gets template for page 
$cur_template = $mainframe->getTemplate();  
// add this 
if( $mosConfig_multilingual_support ) { 	
$cur_template .= "-". $iso_client_lang; 
}
 else { 	
$cur_template .= "-en"; 
} 

Now every template would be named with his standard name plus a "-en" or "-de" or anything else.