Although this may sound like a cop out, if Narnia did not undergo a linguistic Babel (Genesis 11) the linguistic principle of all peoples understanding one another may still have been in effect in this magical world (c.f., Tooky's suggestion mentioning an absence of Babel in Narnia).
I think of the scene in Charn, in which the words inscribed on the pillar on which the golden bell and hammer were written in some unknown script. Yet as Digory and Polly looked at the message they found they could understand it after all (and they also understood Jadis also after the ringing of the bell woke her from her enchanted sleep).
' "There seems to be something written here," said Polly, stooping down and looking at the side of the pillar.
"By gum, so there is," said Digory. "But of course we shan't be able to read it."
"Shan't we? I'm not so sure," said Polly.
They both looked at it hard and, as you might have expected, the letters cut in the stone were strange. But now a great wonder happerned: for as they looked, thought the shape of the strange letters never altered, they found that they could understand them.'
Although their understanding the foreign script may be explained by enchantment, the principle of being able to understand other peoples when the children of Adam and Eve visited may have also been a property of deep magic, an effect of breathing Narnian air, perhaps?
(And as a side note, I expect that in the future in heaven, linguistic barriers will again be broken down so that we may all worship together in our own languages yet all be able to understand one another.)
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Lucy Took wrote:I think they would call it Narnian, just as we call English English, not Anglo-Saxon-Norman. Somehow America hasn't started calling American English American, but we might if England wasn't a place that we could go.
And that is an interesting question about what Caspian was speaking. Perhaps the high born of Non-Narnian countries had distinct dialects by the time Telmar took over?
While linguistic evolution didn't seem to take place in Narnia (Must have driven Tolkien mad ) one reason for there only being one language and a slowing of the language changing would be that there's no equivalent of the tower of Bable in Narnia so there would be no reason for there to be a massive language shift due to mingling with people that spoke something different.