I think we'd all agree that airlines can't do the announcements in all human languages. Even if we only did the ones spoken by a million or more people, or ten million or more, or even a hundred million or more, it would take far too long. There are simply too many languages - in all of these categories.

On the other hand, if a flight attendant speaks a third language and it's clear that some passengers might find it beneficial for annoumcements to be made in that language, why wouldn't they proceed in that language? Sounds like it's simply providing good customer service.