Plugin for custom url management and rewrite fixes
Add app/etc/routes.xml with following config:
<config>
<default>
<checkout_cart_index>
<name>checkout/cart/index</name>
<route>koszyk</route>
<defaults>
<module>checkout</module>
<controller>cart</controller>
<action>index</action>
</defaults>
</checkout_cart_index>
</default>
</config>```