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?

Eventually I grew tired of having to explain over and over why all my class material is in English. I want to put some warning in my syllabus to minimize any arguments. The problem is how to do it politely, yet clearly.

The reasons are clear: there is no material in their native language for the courses I teach; obviously I cannot and will not translate the material; and it is their duty to read in English. I guess I cannot tell them straight that if they do not read English well they will not even understand classes well and will get low grades. I don't want to sound rude.

Is there a way to explain the situation, or is it better not to include such a written statement in a syllabus and keep things as they are?