I would say to write all the foreign language in italics, and let the reader have their aha moment later when they figure out that the reason the prologue was in italics is that it was spoken in Alpha Centaurian.
You may be overthinking this. If two characters share the same native tongue (and know that) of course they most likely will be using that one in their conversations¹
Now, there is an important piece of information added in a comment to Sciborg:
I don't want the readers to question why they're not understanding anyone if they were clearly speaking English in the beginning.
So I guess you want to write some gibberish as that character text later on, e.g.
As Alice exited the underground, she almost got a heart attack when she abruptly encountered a three-legged "person" with glowing blue skin which said: Jbhyq lbh zvaq vs jr noqhpgrq naq qvffrpgrq lbh va beqre gb vzcebir bhe xabjyrqtr nobhg lbhe fcrpvrf?
If you change the POV on each chapter, it may come as natural that it shows only what each character itself understands. Perhaps with just a few readable words that they were able to pick up, and even corrupting them from its own translation. This could lead to funny mix-ups, such as the foreign character concerned hearing
vagrepbaarpgrq vf irel jryy London 2 the Colosseum and make mincemeat of gnxr gur ghor and punatr vagb gur blue yvar hagvy strike king
from the well-intentioned indications of the British character simply telling them to take the blue line from Piccadilly Circus to Kings Cross.
However, this approach could be quite restrictive on the dialogs you are able to produce. Plus, the bigger issue on how the different characters are going to interact.
I don't know the kind of content you want to include in the prologue; given the movie image, it may be some kind of introduction to the later events. If it is amenable to a written form, that may be a better form for which to convey its language.
Instead of a dialogue, your prologue could be a letter to a relative, maybe telling new events, or informing a (grand)child not to forget their ways. That might even include some auto-dialogue ("If you were here, you would probably tell me X but…") if not much is needed.
The prologue could also be part of the character Memoir (maybe even your whole book could be construed as their memoir?), retelling how that dialogue happened decades ago before earthlings were discovered.
Then you might simply end with:
Document 1e0074bd-9c0c-4f4b-ad1c-161b4123c170 translated from Alpha Centaurian by GalaxyTranslator 77.0.Ψ build λ.ς.2 in 0.12ns
Or for a more classical approach:
Letter 45 from Jor-L to Kal-El, Messidor 22th 851 CE
Compiled and confirmed accurate to its original Alpha Centaurian form by Janov Pelorat PhD
¹ The exceptions I can think of is if they want a third party to be able to peek in, if it is a technical matter best referred to in that language, or they just want to practice it.