"It is not often I speak English."
Translation:Det er ikke ofte jeg snakker engelsk.
It could be because the reason you provided (as "that" was not in the English sentence either), but if that's not the reason then it's probably because of your word order. The second part of the sentence is its own subclause in which the verb should go second I think (V2-rule): "...(at) jeg snakker engelsk". To be sure I'll ask a mod to confirm ;)