I have a questions. If I am foreigner, (15 years old, means I already recieved ID card) do I still need Cycling card? If do, are there dome ways to gain it? Or it is just formality, which isn't useful?
According to the rule of law in Poland, people between 10 and 18 years old are obligated to have the bicycle card, when driving on a public roads.
You can get it in a primary school or in a training center for drivers.
If you drive safe, the likelihood that somebody will check your licence is not big, but in the case of any accident or something like that - that's the other story.
Here is a Polish site which answers your questions: