Hen was first mentioned by Swedish linguists in the mid-1960s, and then in 1994 the late linguist Hans Karlgren suggested adding hen as a new personal pronoun, mostly for practical reasons. Karlgren was trying to avoid the awkward he/she that gums up writing, and invent a single word "that enables us to speak of a person without specifying their gender. He argued that it could improve the Swedish language and make it more nuanced.

The whole article: here

I guess there is a law in Sweden for enforcing this word to be used.