Show something when a translation is missing

[aabounegm]

Currently, when there is no translation for a transition it just shows "[❌ Transition]", and messages with no translation just don't show up. This could be mitigated in one of many ways:

• Use the base translation (original language the story was written in) as a fallback
• Make it more obvious that a translation is missing (like a message saying "This message has no translation. Please report the error to the authors")
• Don't display stories with missing translations on the homepage

[aabounegm]

The first is difficult to do since we do not store any information pointing to the intial/base translation.
The last one is also difficult for the same reason and for diffing between the translations.
The 2nd way is chosen as a temporary measure