Added remaining translations to feature custom transaltion strings on Admin Panel.

Thanks to xet7 !

imports/i18n/data/en.i18n.json

@@ -1230,8 +1230,11 @@
   "invalid-file": "If filename is invalid, upload or rename is cancelled.",
   "preview-pdf-not-supported": "Your device does not support previewing PDF. Try downloading instead.",
   "drag-board": "Drag board",
-  "translation-number": "The number of custom strings is: ",
-  "delete-translation-confirm-popup": "Are you sure you want to delete this custom string? There is no undo.",
+  "translation-number": "The number of custom translation strings is:",
+  "delete-translation-confirm-popup": "Are you sure you want to delete this custom translation string? There is no undo.",
+  "newTranslationPopup-title": "New custom translation string",
+  "editTranslationPopup-title": "Edit custom translation string",
+  "settingsTranslationPopup-title": "Delete this custom translation string?",
   "translation": "Translation",
   "text": "Text",
   "translation-text": "Translation text"