Hello Lech,
As much as I would love to help you further, I am not the developer of WPML.
You should contact their support team for further assistance.
Keep in mind there are different elements to translate (ie: Widgets, Pages, Posts and Settings).
So make sure you are looking for the correct strings when trying to translate the remaining strings.
Here is the link to their support site, https://wpml.org/forums/forum/english-support/
Hopefully they will be able to help you further.