Do I have the legal right to demand information on plagiarism of my work from another university?
In my opinion, what seems to have gone wrong is that the examination office employees do not have an interest in pursuing this issue. After all, the student having handed in your thesis may sue the university if the thesis gets rejected. After the degree has already been awarded to the student, it is quite difficult to retract the degree. This is probably why they asked for an official translation. By the way, the examination office does not rate or examine anything - they are concerned with the administrative processes of examinations, and are thus probably unable to deal with this issue correctly. They might have felt that they need to deal with this issue, though, which is why they asked you to provide an official translation. As you wrote, you shouldn't be expected to pay for this.

As a suggestion, you may want to consider contacting researchers at the university directly. Point of contact number one should probably be the advisor of plagiarizing student. Other points of contact may be the secondary examiner, or the dean of studies. Provide them with bibliographical information about your thesis and a PDF of your thesis. At larger universities, there may probably be Russian PhD students that they can ask for a first look. In the east of Germany, it is not entirely uncommon that older professors can read Russian themselves. Once they have some indication that your accusation may be correct, they are likely to arrange for a closer check.