Add missing translation

config/locales/bg.yml

@@ -116,6 +116,9 @@ bg:
             picture:
               invalid_content_type: "невалиден формат на снимката"
     models:
+      volunteership:
+        one: доброволец
+        other: доброволци
       conference:
         one: "конференция"
         other: "конференции"

config/locales/en.yml

@@ -112,6 +112,9 @@ en:
             picture:
               invalid_content_type: "invalid picture format"
     models:
+      volunteership:
+        one: volunteership
+        other: volunteerships
       conference:
         one: conference
         other: conferences