Good question - they speak English to an extent but it's a little mangled:

'Where?' Amherst shook the native's shoulder excitedly.

'In Iticht phulph.'

'In locked valve,' Amherst shouted. 'Of course. It's the only place he could be.'

I'll have to see if I can work out the linguistics in more detail, there aren't many examples of speech.

And I've just noticed that about 95% of the way through the story we're told that the Nympus have green scaly shoulders. At least I got the scaly part right, the green will have to be looked into...