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

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.