It looks like I can handle it. It's perfectly easy.
I read the Wiki entries on Transit, so I fully expected to do precisely that. But the .NLD files are already there.
That sort of looks OK. But then there are files (both .ENU and .NLD) that trigger the error message as shown above, and there are _AEXTR files that show wonderful characters that don't seem to have anything to do with languages as we know them.
Ask a nice person to export the language pairs as a TMX file in Transit. Translate the .ENG (or .DEU) files. Rename afterwards to .NLD.
My first Transit project in more than 20 years of translating. It's a mess. I don't know what files to translate, which ones to align (?), and more. Like:
This has nothing to do with my CT license. The client's Transit copy?
I prefer https://www.youtube.com/watch?v=nbnSllAQl_A