I just know ProQuest which supplies some PhD dissertations. However, it's hard to find some the dissertations of French and German.
So are there any other good databases for dissertations of French, German, UK, USA? Any answer is encouraged.
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For French dissertations, you have theses.fr which lists both the on-going and defended thesis since 1985 in France.
Note that there may be dissertations that are not available online but are of public access in the university libraries. Also, most of the theses are written in French (~270k in French, and ~23k in English), so you will probably need decent French skills if you find a thesis that matches your interests.