Translation:She speaks not only English, but also Dutch.
The English translation provided stated, 'She does not only speak'; however, the 'only' should not qualify speak, but rather English.
In the since of L1 learning, 'ook' sometimes better translates to 'also' than 'too'. In the sense of L2 the proper way to frame things when using descriptors like this one follows the same rule that 'also' follows, but when it is used in any case where 'as well' might be used, it will follow the 'too' rule.
Ik leer Nederlands, maar ik leer je ook. I am learning Dutch, but I am ALSO teaching you. (Teaching you as well)
Ik ben Nederlands aan het leren, en jij bent ook aan het leren. I am learning Dutch, and you are ALSO learning. (you, too, are learning)
Does this cover what you needed to know?
I think "ze" should be accepted, it means "she" or "they" and I also learned that "zij" (and "jij", "wij" etc) are to be used to indicate stress on that word. Can anyone explain why only "zij" is accepted here?
If you're listening to an audio exercise and the speaker says "zij", then "ze" is a wrong answer.
THe word order is really hard to find ! better if I have any refference .. grammar
If you log into Duolingo through the web version (instead of the app), you'll find the Grammar Tips and Notes at the beginning of each skill.
You can also search on the discussion section (again, web version), since there is a user who compiled all the grammar tips and notes of several Duo language courses and shared them there.
Additionally, you can use websites like dutchgrammar.com -I find the explanations that are provided there to be quite clear.