It's a plot hole
I've gone through the transcript and I can't find any explanation. The writers in Farscape usually call out translator logic pretty clearly, so its absence seems like an oversight.
A talk conversation on wikia agrees:
The beings in "I E.T." either had no TMs and the writers screwed up because it was such an early episode, or they too had TMs left brought to the planet by aliens long ago, causing the TMs to spread throughout the general population up tot the point that they were so used of understanding everyone around them that it never occured to them that it was pretty weird they could instantly understand the alien Crichton.
However considering that this is not only a planet without advanced technology - but that this is a first contact scenario, coming up with a plausible in-universe explanation is probably pure speculation. At the basics it would seem like either they either had TM's as the wikia suggests, or some kind of technology which mirrors the TM's in functionality.
The normal wiki episode page also notes the plot hole:
Despite not having translator microbes in their body the native population of the planet in this episode can understand Crichton and the rest of the Moya crew.
And the best explanation is indeed that is was an early episode:
O'Bannon admits that while the episode suffered in execution because it was filmed so early in the series, he stands by the script since it keeps John down-to-earth and serves as a touchstone for all the episodes that follow.