A Certificate of Accurate Translation may be provided upon request. See a sample certificate here.
The certificate is included free of charge for all expert-level translations. For 'business general' translations the certificate can be included for a fee of $5.
To receive the certificate simply check the "I need a certificate of accuracy" box when submitting your project via the translation Wizard.
You can also request the certificate directly from the project page, once your translation is done.