Ok, just tested it :
- installed 'internationalization' module and enabled only 'internationalization' (formerly 'i18n')
- activated 'language switcher' block
- added this code in settings.php :
$conf['i18n_variables'] = array(
'uc_teaser_add_to_cart_text',
'uc_product_add_to_cart_text',
);Then, when I go to product settings, the language beeing set to english :
- the two 'add to cart' textfields are empty.
- the sentence This is a multilingual variable is added in each textfield's description
- enter 'Add to cart' twice, then submit
- switch language
- the textfields are empty
- enter the translated string twice, then submit
- go to a product node, and switch language : the 'Add to cart' button is translated accordingly
So, it works. But a drawback is that, as soon as you make a variable translatable, the default value is ignored (I had to enter 'Add to cart' again).
will investigate this later 
Joined: 02/06/2008