How can I change the language of the quarantine reports?

The language of the Quaratine report is based on the globally configured language of a partner, customer or user account.

If a language is configured on a partner basis, it is inherited by the customers and users.
This inheritance can be customized on a domain (customer) and user basis.
For example, if the global language of a partner account is set to English, it can be changed to German on a domain basis. This setting will then apply to all users of the domain.
If a user again prefers a different language, this can be adjusted on a per-user basis.

The corresponding settings can be found in the manual.