What is the ruling on translating university theses for students when there are doubts that they are plagiarized? And what are the cases in which this is permissible?

The basic principle is that translating messages is permissible, unless you become certain that the student cheated in them. In that case, translation is not permissible, because it constitutes aiding in sin. However, mere doubt does not necessitate prohibition; for doubt is like non-existence, it does not remove the matter from the original permissibility.