The reason not all of the text is translated is because not every text string in the app has been set up to work with the internationalization code. This is an area that we need to improve upon, so thank you for the feedback. Once we set up the remaining text strings, those will need translating as well.
In the meantime, if a translation does not exist then the system will fall-back to English. Thank you @William for pointing out that this can make following the English software documentation very confusing and difficult if the app is partially (or fully) translated. Based on your feature request, we have added to our roadmap a user setting for selecting your language. In the meantime, you should be able to change your system language settings so that it tells the app that you speak English so that it displays fully in English. I understand this is a work-around though, but its probably the best option right now until we have more robust internationalization options in place.