A message's translation will vary in duration, specially if we are talking about a human-translated oubound message. Bigger messages will take longer, as will messages with content that is more difficult or specialized in nature. Submitting source text containing errors or of poor quality also increases the turn around time for a translation!
Still, if you find that the translation is taking much longer than what it is supposed to, and you are seeing the Translating... message for too long, something might have gone wrong.
The strongest possibility is that a bug affected the translation request. If this is the case, we advise you to report the issue to email@example.com, identifying the message UID and/or Ticket ID (label).