Doctor Who musing

How does the TARDIS translation work when the Doctor and companions are around a multilingual group of people? I mean, I suppose when addressed as a group they all hear whatever language they all have in common, or the one most likely to be used as lingua franca in the setting; but if the Doctor takes one person aside, do they perceive it as a switch to their own native tongue? Or are their assumptions (about where the Doctor comes from) a factor?