Add english translations to make fallback work

Author: carlobeltrame

frontend/src/locales/en.json

@@ -387,7 +387,18 @@
           "label": "Download PDF (layout #2)"
         },
         "generatePdfMixin": {
-          "error": "Something did not work. Please try again, or try reloading the page."
+          "error": "Something did not work. Please try again, or try reloading the page.",
+          "progress": {
+            "loadingData": "Loading the latest data of the camp…",
+            "setupTranslationsAndFonts": "Loading fonts and static texts…",
+            "assembleContent": "Assembling {type} ({content} of {totalContents})…",
+            "layoutDocument": "Calculating document layout…",
+            "layoutPage": "Calculating layout of page {page}…",
+            "renderingPdfPage": "Rendering pdf page {page} of {totalPages}…",
+            "downloadingPdf": "Downloading pdf file…",
+            "done": "Done",
+            "failed": "Failed"
+          }
         },
         "printPreviewClient": {
           "previewError": "Something did not work. Please try again with different printing settings, or reload the page.",