The "Delete" button for files in a project should be clickable, whether they are reference or translation file - at least for administrators or project managers.

If you accidentally add the wrong file to a project, you cannot delete it. This is also the case if you notice that the file still contains a hard return, for example, that needs to be removed for segmentation purposes.
Additionally, files are sometimes updated shortly after being added to the project. As it is not possible to upload files with the same name, the updated file must be given a different name, which can lead to confusion in projects with many files.
Even when cancelling files, you still have to rename the modified file first. This means that the 'old' file version remains in the project unnecessarily.