In your Japanese airport example I think they're intentionally not the same phrase. "Welcome back" to the Japanese people and "Welcome to Japan" to the foreigners.
Re:An interesting Google Translate quirk
by tr4nslator on Tuesday October 21, 2008 @06:41PM
(#339469)
Yes, that is why it's not a translation, but a localization (you know, for the locals).
In this case, Google's approach confuses the latter for the former, because its corpus contains parallel texts that have not only been translated (into a different language), but also localized (into a different locale). For example, it may be extrapolating incorrectly from a French document and an English document with the following phrases in the same location:
This sign is another good example of how parallel texts are not always translations:http://flickr.com/photos/jadelin/2205088201/
(the Japanese "おかえりなさい" here means "Welcome home")