UNESCO produces a resource called the Index Translationum, an international bibliography of translations. It's just been added to the E-Resources site.
The database contains cumulative bibliographical information on books translated and published in about one hundred of the UNESCO Member States since 1979 and totals more than 1.800,000 entries in all disciplines: literature, social and human sciences, natural and exact sciences, art, history and so forth. It is planned to update the work every four months.
The Index has been published since 1932. Widener has volumes not covered in the online version at these call numbers: Widener | WID-LC | PN886 .Z99 I52x , Widener | WID-LC | PN886 .Z99 I53x.
So next time someone wants to know what Dutch, German, or English versions of Tolstoy's War and Peace may be out there, you'll have a place to go!