Why is this not 'oni'. It was my understanding (probably learnt wrong) that 'oni' is if you are talking about one or more masc. in the group, and 'one' if they are female?
yes, if you are talking about people: One is a group that has only female people in it Oni is a group that has at least one male person in it.
Both one and oni translate to They. They speak can be either "oni mówią" or "one mówią"
Why "They are speaking English" is correct? Why not "They are speak English"?
You tried to use Present Simple and Present Continous at the same time. It's either "They speak" or "They are speaking".