Worldwide Dissertation Database? or French, German, Italian ones?
I am not aware of an international database, but basically all german dissertations have to be submitted to the German National Library (Deutsche Nationalbibliothek), so if they exist in physical form they can be accessed at the libraries in Frankfurt and/or Leipzig. If they only exist in electronical form, they are still registered at the library. The following link is going to give you a list of all german dissertations that are freely available online:
DNB search

The DNB also has a manual about searching for dissertations, however I could only find a german version that is available here