You ask why DIN is commonly found in English, and UNI not?
Having done an awful lot of professional work using standards, I would suggest that this is mainly because the DIN standards are about the only ones that are EVEN MORE strict than the BS [=UK] ones --- hence they are often referred to in conjunction with these. The other national standards, like NF or UNI, crop up much less often in a UK context, and in many cases, I suspect, may well in fact be based (to a greater or lesser extent) on their BS or DIN equivalents. It is very noticeable the way certain countries' standards often provide the yardsticks for the others in certain specific fields.