

Welcome to everywhere. 3.5" disks in German are called “dreieinhalb Zoll Disketten”, and in Dutch “drie punt vijf inch floppys”. Both of those translate roughly to “three and a half inch disks/floppies”. Everyone borrowed US computer terms and translated them directly.
No country uses the metric system exclusively. None. You will find exceptions if you look for them. This isn’t some kind of moral failing, it’s just practicality. Look at how car tires are sold for one example that’s nearly universal due to industry standards.
Instructions unclear, now sitting in basement.