There are different ways how to use Phoca Cart in multi language mode.
Basic component, module and plugin strings are standard Joomla! language strings which can be translated by Joomla! INI files (for example: de-DE.com_phocacart.ini).
If you need to translate content strings (products, categories, ...) there are different ways how to achieve it:
1) Using default Joomla! feature - this means that each language will be applied for each item. For example products: One product becomes two or more for each language. So Product 1 will be stored as two products - one for English language, second for e.g. German language. The same way like Joomla! articles are stored.