Add translations for actors and sources in backend

backend/en/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": "\"Backgrounds\" refer to contexts for objects. They refer to the standard data of museum-digital"
+        "about_backgrounds_text": "\"Backgrounds\" refer to contexts for objects. They refer to the standard data of museum-digital",
+        "license": "License",
+        "license_explica": "The license of the source or the digitized version thereof.",
+        "source_fulltext": "Full text"
     }
 }