How can I politely explain to my students that the texts in my course are all in English and it is their duty to read it nonetheless?
I agree that it makes sense to set expectations as early as possible.

I'd just put


  Note that all XXX for this course are in English.


in the syllabus. You should of course be specific as to what XXX is:


Class slides
Required reading
Supplemental reading
Lectures (you could have slides in English but speak the local language during the actual lecture)
Quizzes
Exams


If quizzes/exams are in English, you should also note whether students would be expected to answer in English.

Whether or not you also want to include the reasons for this is really up to how much space you have in the syllabus and whether there isn't anything more important to put there. I'd say that a warning might be a better use for the space:


  If your English is not up to reading/understanding/writing technical documents, this course likely is not a good fit for you.