"Dit is de langste rivier van Duitsland."
Translation:This is Germany's longest river.
Doorluchte Rijn, mijn soete droom, Van waer sal ik u lof toesingen. Mijn treckende geboortestroom, Ghy koomt uit Zwitsersche Alpes springen ... (Joost van den Vondel, "De Rynstroom", 1629/30, in the original spelling)
How do you say in Dutch "This is the longest river from Germany", meaning the longest river in our country coming out of Germany?
Yes, only a few countries are mentioned with the definite article: the United States, the United Kingdom, the Netherlands are exceptions, and there are probably others. In German, however, one says "die Schweiz" (French: la Suisse; Italian: la Svizzera), but in English only Switzerland.
Ok I understand! So, just stay that sentence without The : "this is the longest river of Germany" was right???
It is not actually wrong, but one would probably say (in addition to the translation given above): "This is the longest river in Germany."