You should set the Language property in the ConfigFile.xml / ListConfigFile.xml files:
In order to make the localization to work you need to copy the en-US files from Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\x.x.x.x__1f131a624888eeed\Resources\App_GlobalResources folder to \Inetpub\wwwroot\wss\VirtualDirectories\80\App_GlobalResources folder as well as the Polish ones in it too.
You should make sure that the Polish resx files contains all localization strings available in the en-US ones and that they are up to date.
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