Is it ethical to reference a work accused of plagarism?
First, you need to decide if you find the plagiarism charges credible.  Assuming that you do (the material at the link you include certainly looks damning), then you certainly should not give any citation to the plagiarists.

But that still leaves a dilemma: if you can't read the language of the original work, can you honestly cite it?  To think about how to solve this, let's return to the principles of what a citation actually accomplishes:


A citation justifies an assertion in your work
A citation gives appropriate credit to the related and supporting intellectual work of others
A citation gives readers links to follow in learning more about the subject.


If this reference has been helpful to you, then the original author deserves citation on the basis of purpose #2.  That does not, however, support purpose #1 or #3 if neither you nor most of your readers can read the language.  The ideal solution would be if the original author has material with similar content, but written in English, which you can cite instead.  Switching citations like that still fulfills all the criteria.

What if there is no appropriate source in English by the original author?  If you can verify that the original work really does contain the parts that you find useful (e.g., via online translation or a friend who speaks the language), then cite the untranslated original, fulfilling purpose #1 and #2.  Then, since this is textbook-level background material, there should also exist some non-plagiarized English works that are appropriate background to cite: find them, and add them in as well, strengthening purpose #1 and fulfilling #3.