Alienation

If a human population, originally from Earth, had lived on another planet for tens of thousand of years, then they would by now have become strangely foreign or alien to Terrestrials in psychology, language, behaviour etc - as alien as some of the fictional aliens in works of sf. SM Stirling shows this clearly in In The Courts Of The Crimson Kings. 

Decades ago, an sf comic strip set in the future showed a meeting of what looked like standard fictional aliens. However, a footnote explained that these individuals with differently shaped and hued bodies were descendants of human colonists of extrasolar planets, physically changed by their mutually diverse and alien environments.

Sf writers need to make such differences of speech, culture etc, if not yet of body shape, clearer when describing e.g., in Poul Anderson's case, Hermetians, Avalonians and Dennitzans. These are human inhabitants of planets called Hermes, Avalon and Dennitza as opposed to Wodenites, Cynthians or Ythrians who are natives of Woden, Cynthia and Ythri.

On Avalon, jointly colonized by human beings and Ythrians:

Planha pronunciation influences Anglic pronunciation;

when Christopher Holm speaks as Arinnian of Stormgate Choth, his accent diverges even more and he sounds as if he is translating directly from Planha;

Terrans find it difficult to spy on Avalon because they cannot imitate Avalonian human thought, speech or even ways of walking.

But more of these differences need to be evident to readers. On Aeneas, there are noticeable differences in speech patterns between Nords, tinerans and Riverfolk, all human.