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.