DPD allows you to set the default language to be displayed in your cart. This setting only affects non-customized text so if you've added any extra text to your DPD pages via the Theme editor you will need to manually translate that there.
Here is a list of DPDs supported languages:
To select the default language for your cart click Preferences in the left menu of your DPD dashboard and you'll find the Default Language field in the first block of fields as shown in the screenshot below:
Once you've selected your default language be sure to save at the bottom of the page to have this change apply.
Your customers will also be able to select the cart language during checkout using the dropdown list shown below: