it seems that I've got 1,5gb of data in internal (system data) - how can I delete?
The internal-storage partition is formatted when you execute a factory reset.
According to the AOSP sources of the disk usage settings app the category
Everything else that hasn't already been attributed.
In concrete the value is calculated based on the "Others" value minus the data occupied by apps that belong to the the other displayed categories.
This makes it very difficult to say what data is included in this category.
Anyway 1.5GB of system data is typical value for an Android phone. It is not necessary to take any actions.