While it may not be wise to do this it isn't unethical unless grading is competitive in some way. It potentially gives an advantage to those who take it later, but that doesn't necessarily imply a disadvantage to those who take it earlier.
But "grading on the curve" where some students must fail so that others can excel makes the calculation much different. In my opinion, however, that sort of grading is unethical in any case.
While I'm guessing that the exam is translated from one language to the other, that doesn't change the ethics of it, unless it is given to students in a language they can't be expected to know. Then again, it is unethical, as it disadvantages some students whether or not it advantages others.
One reason to question the wisdom of this is that it may change the behavior of students for future exams to their detriment. If they think they will get a time window in which they see the exam then they may not learn the material. This is bad in the long term, of course, but also in the short term if the future exam is changed.
Another reason that such actions are unwise is that the professor and others responsible for the student's education get much less information about the state of knowledge of the students. If students can easily provide answers because they have seen the questions, you can hardly know what they understand or would be able to do in less advantageous circumstances. If you believe, as I do, that one of the main purposes of testing (number one in my reckoning) is to know how to further guide the progress of the course and its individual students, then you have lost an important tool and indicator.