I am going to tentatively change the title back to Legends of Valhalla: Thor, unless someone can point out where I have erred because IMDB and the official site are using the title Legends of Valhalla: Thor.

As per IMDB, the title it is

Legends of Valhalla: Thor (2011) Hetjur Valhallar - Þór (original title)

As per the films official website, the title is also referred to as Legends of Valhalla: Thor. Eg. "The making of Legends of Valhalla: Thor"

I will add Thor: Legend of the Magical Hammer to alternative titles.

UPDATE: I strongly suspect IMDB is displaying different default titles that correspond to a users IP location. For instance, American IP addresses are probably showing "Thor: Legend of the Magical Hammer", while non-American IP addresses are displaying the international English title, "Legends of Valhalla: Thor". The entire list of country specific titles can be viewed here.