fix(watch): a link whose video is being made again says so, in the page's language

The public watch route answers video_preparing for an asset that is pending or
downloading, and the page shows "being prepared — try again" with a Retry
instead of "this link is gone". watch.html ships one lang; the page now sets
documentElement.lang to the language its strings were picked in.

frontend/src/i18n/locales/en/errors.json

@@ -191,6 +191,7 @@
     "unknown_setting": "Unknown setting: {{key}}",
     "value_missing": "A value is required",
     "video_gone": "This video is no longer available",
+    "video_preparing": "This video is being prepared. Try again in a few minutes.",
     "view_name_taken": "You already have a view named “{{name}}”",
     "watch_sync_off": "Turn on Plex watch-sync first",
     "weak_password": "The password must be at least {{min}} characters",

frontend/src/i18n/locales/hu/errors.json

@@ -191,6 +191,7 @@
     "unknown_setting": "Ismeretlen beállítás: {{key}}",
     "value_missing": "Adj meg egy értéket",
     "video_gone": "Ez a videó már nem érhető el",
+    "video_preparing": "A videó előkészítése folyamatban. Próbáld újra néhány perc múlva.",
     "view_name_taken": "Már van „{{name}}” nevű nézeted",
     "watch_sync_off": "Előbb kapcsold be a Plex megtekintés-szinkront",
     "weak_password": "A jelszónak legalább {{min}} karakterből kell állnia",