Hi, Waseda only offers business/commerce undergraduate courses in Japanese. If you have forgotten all your Japanese (ie. starting from the very bottom again), you might NOT want to try it because basically 1) all lectures, tutorials and discussions are in Japanese; 2) All homework must be in Japanese. But if you are somewhere half-way then taking intensive japanese classes (which incidentally, is offered by Waseda University as well) might help.
Most of the international students who want to come to Japanese universities for undergraduate studies in Japanese in general takes at least 1 year of full, intensive japanese language classes (ie. just Japanese everyday). Then at least you should be able to converse, read and write. One way to gauge yourself might be taking the JLPT exam. I was told that you at least need a 3 or 2 to get through the classes. FYI.