feat: Translate frontend/src/i18n/en.json in bg

99% of minimum 50% translated source file: 'frontend/src/i18n/en.json'
on 'bg'.

Sync of partially translated files: 
untranslated content is included with an empty translation 
or source language content depending on file format

frontend/src/i18n/bg.json

@@ -266,7 +266,7 @@
     "days": "Дни",
     "hours": "Часове",
     "minutes": "Минути",
-    "seconds": "Seconds",
+    "seconds": "Секунди",
     "unit": "Time Unit"
   }
 }