What to do about a Master's thesis where most of the introduction is a literal translation of my own work? I have just discovered the introduction of an Engineering Master's thesis defended some months ago in my ancient university has been made mostly by "cut &amp; paste" (and literally translate from English to Spanish) part of a book chapter I wrote some time ago. About 80-90% of 10 pages have been copied verbatim from my work.

It is true the author of this thesis cites my work when he copies text without any reference in the original. But since he fails to include such sentences (or any other copied and translated text) as quotes, it seems as if he originally wrote those sentences. The order of the sentences has been somewhat modified, but there is no paraphrasing.

I understand this constitutes plagiarism and I would like to do something about it. I do not intend to undertake legal action or to report this to the university (I do not think it has any normative on the subject, anyway). So, what could I do? Writing the author and explaining him the seriousness of what he did and why? Warning the advisors (whom I know well) about it? Your suggestions will be much appreciated.
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.