Added translation to support viewing PDFs.

Thanks to xet7 !

Fixes https://github.com/wekan/wekan/pull/4989#pullrequestreview-1507053336

client/components/cards/attachments.jade

@@ -34,8 +34,8 @@ template(name="attachmentsGallery")
   #viewer-overlay.hidden
     #viewer-container
       object#pdf-viewer(type="application/pdf")
-        span.pdf-preview-error Your device does not support previewing pdfs. Try downloading instead.
-    a#viewer-close.fa.fa-times-thin
+        span.pdf-preview-error {{_ 'preview-pdf-not-supported' }}
+      a#viewer-close.fa.fa-times-thin
 
   .attachments-gallery
     if currentUser.isBoardMember

imports/i18n/data/en.i18n.json

@@ -1219,5 +1219,6 @@
   "allowed-upload-filetypes": "Allowed upload filetypes:",
   "max-avatar-filesize": "Max avatar filesize in bytes:",
   "allowed-avatar-filetypes": "Allowed avatar filetypes:",
-  "invalid-file": "If filename is invalid, upload or rename is cancelled."
+  "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."
 }