Add translations for actors and sources in backend

backend/pl/sources.json

@@ -60,6 +60,9 @@
         "entity_of_origin": "Entity of origin",
         "entity_target": "Target entity",
         "about_backgrounds": "About the Backgrounds Section",
-        "about_backgrounds_text": "A text about the backgrounds section will appear here."
+        "about_backgrounds_text": "A text about the backgrounds section will appear here.",
+        "license": "License",
+        "license_explica": "The license of the source or the digitized version thereof.",
+        "source_fulltext": "Full text"
     }
 }