BuddyPress.org

Did you follow the instructions on this page: https://pl.buddypress.org/home/ ?

You may also want to make sure you have the language specified in your wp-config:

`
/**
* WordPress Localized Language, defaults to English.
*
* Change this to localize WordPress. A corresponding MO file for the chosen
* language must be installed to wp-content/languages. For example, install
* de.mo to wp-content/languages and set WPLANG to ‘de’ to enable German
* language support.
*/
define (‘WPLANG’, ‘fr_FR’);
`

change to Polish in your case, of course!