First, I love Danish. Second, this is the perfect sentence to show someone who isn't learning Danish, and count how many consonants AREN'T pronounced.
Suddenly makes it really easy to understand how the English may have adopted 'will', doesn't it. Perhaps really they adopted 'er ved at' without knowing it - in this sentence it sounds almost the same.
I think it is going to rain
... should be accepted.
Because that is bad English.
The form is "about" + infinitive: "about to rain", "about to cry", "about to erupt".