The filenames in the DVD images appear to be Translation-??.gz, but on the repository website it's .bz2 for most files, and .xz for bookworm Translation-en.
A possible workaround I have not tried yet is downloading Translation-en.xz from the repository path corresponding to the .iso images (the URL is in the .jigdo files), and reprocessing it into the missing Translation-en.gz. I extract the DVD pool and dist files and remake them into a local repository-style directory on a local disk that I can use with apt locally. But it will of course not fix the images.