This looks like something related to a useful training course I took a few years ago … didn't go any farther with what I learnt, and now I find I didn't tidy up after myself: my bad. (though I'm fairly confident I'll have backups of anything I changed)
symptom: trying to open DITA files in FM12 (structured interface) tells me "Translator client (ditafmx_app) was not found. Operation could not be completed." Fair comment.
question: which config file do I need to edit, and what should it say if I want to use out-of-the-box FM?