The -sen ending looks more Scandinavian than Dutch to me ... in the 1600s I would expect a Dutch surname to be something like Frederiksz, with the z being the initial of zoon = son. But no -sen name that I've ever seen has been Dutch.
And wherever Latin was known, people Latinised their own names and the names of others, especially when entering them on official lists and forms. So, although Herman is Dutch and Hermann German (very broadly), people speaking/writing both those languages would use Hermanus when, say, registering a birth. And in some cases they would actually also pronounce it Hermanus and use it like any other vernacular name.