[dghf]

But implied translation into the language of the audience is a long-standing convention. We don't watch Ben Hur or Spartacus or I, Claudius and wonder why the Romans are all speaking English instead of Classical Latin.

But it might jar to hear a group of aliens natively speaking Dutch, for example. (Unless it's a Dutch film for a Dutch audience, of course, in which case it might equally jar to hear them speaking English.)

[kergonath]

It was commonly done at the time by having people with different accents. See the queen’s Latin trope. It would not be any weirder intrinsically, we’re just not used to it.

[dghf]

Well, that's fair.

> people with different accents

Done to (arguably) comic effect in the BBC sitcom 'Allo 'Allo!, about the French Resistance in WW2, where English, French and German are all indicated by outrageous caricatures of the relevant accents.