When you create a notebook, you have to define a recognition language: Nebo will expect you to write in this language. Once a recognition language is set for a notebook, it cannot be changed later on. However, since English is very popular, it is recognized in any case and in addition to any language you define.
As an example, if you want to write content mostly in Polish and some in English, choose Polish as the recognition language and both Polish and English should be recognized. But if you want to write Polish and German in the same notebook, it is not possible at the moment.
Note that the choice of the appropriate recognition language is really critical to get an excellent recognition.