I suspect "you do not speak English" is the right translation. Not sure how a FInn would say "you are not speaking English" (indicating you are not doing it now).
The negation has no effect in this: Sinä puhut englantia / Sinä et puhu englantia.
If you use the verb ”sanoa” (to say), the form should be ”englanniksi” (in english) If you use the verb ”puhua” (to talk), the form should be ”englantia” (the english language)
”Haluatko, että puhun suomeksi” is kind of correct and will be understood, but at least to me feels a little clumsy. In your example you could say:
”Haluatko, että puhun suomea vai englantia” ”Do you want me to speak finnish or english” or ”Haluatko jutella suomeksi vai englanniksi” ”Do you want to chat in finnish or in english”
Well the -ksi suffix is, at least on paper, the correct one. But some verbs in Finnish necessitate certain suffixes and those take precedence. This is the case in puhua (speaking a language requires partitive, speaking about something requires the elative case (like many other verbs)). This is also the case in the textbook examples of rakastaa that require a partitive, etc.
From a translational point of view, the distinction between "to speak in Finnish" and the more general "I speak Finnish" (as in, you have the ability to do so), does not really exist in Finnish puhua without added context.
This happened to me once. I had been studying in China for two years and was used to all foreign students of my age speaking Chinese. Some would sometimes switch to English, but most of us would still answer in Chinese. Once, three American students asked me for directions in Lijiang and I explained to them how to go. The dialogue went like this:
"Hi, do you know how to get to the youth hostel?"
me: "Sure, we live there, too. You go back to the main street, turn right, and then you will see that large Mao statue. The youth hostel is opposite of it."
"And and do you speak English?"
me "Yes, of course I do also speak English."
"Well, bye then!"
me: "See you!"
And back they went.
And my friend next to me laughed and told me: "Do you realize that you are not speaking English? I had the impression that they did not understand your Chinese."
Yeah, that was the dialogue from my point of view. But I had indeed spoken Chinese and not realized that they did not understand a word of what I said.
You'd actually express if by using this very same sentence. Finnish has only one present tense, preesens, and no future tense, so you could translate "sinä et puhu englantia" as "you will not speak English" as well. :)
But here it's of course a bit weird to translate the sentence as "you are not speaking English" as that translation is not consistent with the previous ones given.