It is West-Frisian (Frysk), and the languages are close, but so are Dutch and German or West-Frisian and German (so not that close).
I'm not getting the behaviour you describe though. It detects West-Frisian and does a passable translation, but if I force it to see Dutch as the source it makes no sense of it at all.
It is also failing quite hilariously on „in smak minsken”, which means “quite a bunch of people”, but gets translated as “a taste people”. Google sees „smak” as a misspelling of „smaak” (taste), but it means “a bunch” or “many”.
You are right: I tried again and I can't reproduce the behavior. Perhaps I got confused and selected Frisian twice, the first time I tried. (Google's UI lists it as simply "Frisian" for me, not West-Frisian).
I think most people call it Frisian. West-Frisian is used formally because other (mostly historic) variants exist.
What is now the province of Frisia is only a modest part of what was once a larger kingdom stretching into the North-East of modern Germany (where Ost Friesland is the name of a region in Niedersachsen still). Confusingly, in the Netherlands there is a West-Friesland as well to the West of the province, but people don't speak or understand (West-)Frisian there.
I'm not getting the behaviour you describe though. It detects West-Frisian and does a passable translation, but if I force it to see Dutch as the source it makes no sense of it at all.
It is also failing quite hilariously on „in smak minsken”, which means “quite a bunch of people”, but gets translated as “a taste people”. Google sees „smak” as a misspelling of „smaak” (taste), but it means “a bunch” or “many”.