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

97% 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

@@ -272,5 +272,10 @@
     "minutes": "Минути",
     "seconds": "Секунди",
     "unit": "Единица за време"
+  },
+  "available_csv_separators": {
+    "comma": "Comma (,)",
+    "semicolon": "Semicolon (;)",
+    "both": "Both (,) and (;)"
   }
 }