As the "Search" string seems to be somewhat special, I've filed bug 567127 separately.
As for the remaining untranslatable strings (Messages, Replies, etc.):
for stream in ["messages", "replies", "images", "private"]:
Perhaps deferred translations and the N_() macro could be used:
The other untranslatable strings I'm not sure where they are shown on the UI:
default_streams = self.services[account["protocol"]]["default_streams"]
Ken, Ryan, is it ok to reopen this bug, or would you prefer a new one to track the remaining issues?