What to do about a Master's thesis where most of the introduction is a literal translation of my own work?
About 80-90% of 10 pages have been copied verbatim from my work.

80 TO 90%?!
This is a significant amount of content. Even translated, I feel like it demonstrates purposeful intent to plagiarize (akin to using Word's auto-summarize feature to summarize an article), rather than paraphrasing an amount of text to show a point. Translating word for word, as per my APA link above, with proper citation, for a single point is one thing, but when the bulk of the introduction is someone else's work, this seems to me as academic misconduct rather than poor use of citations. I mean... EIGHT TO NINE PAGES. It is literally 8 pages worth of your work, translated, as his introduction. It might as well be YOUR introduction, given how much you've put in, and someone else translated it.
Translated books are still accredited to their original author, with a translator tagged. The way this sounds cited, he's basically taking credit for what you've written, citing you as the person who provided information for the idea, as opposed to the person who actually wrote it and facilitated by his translation.
Unless he's cited and mentioned that it's a direct quote translated after every sentence, this seems like willful plagiarism.
I would recommend you talk to the advisers about this issue now that you've demonstrated the scope of the issue. While you may not have a significant vested interest in the outcome of the situation, the advisers definitely will, and they will likely know how to handle the situation. Also, confronting the student in question is not a good idea. The university awards the degree to the student (and the degree's worth is determined by the integrity of how it is awarded), so it is probably in the best interest of the university as well to determine the action resulting from something like this.