new holland

This is my first alternate history I plan to go into a lot of detail with. Basically, to form stronger defence against English colonisation in North America, The Dutch and Swedish agree to merge their colonies peacefully rather than the dutch invading new sweeten during the great northern war...

So, assuming that end up with a Dutch-settled Australia [New Holland] would the population end up forming their own language like the Afrikaner's did or would it just be a local variant of Dutch? Also, if a unique language did form what would it likely be called? Whatever New Holland ends up...

I know that IOTL the British left part of the continent open in case of eventual Dutch interest in it. What I dont know is the point where the British would have not been okay with the Dutch colonizing their half. When is the last possible point that the Dutch could have colonized New Holland then?