While Caesar was campaigning in Britain, he was approached by some of Cassivellaunus' allies who wished to defect and form an alliance with Caesar. If I'm not mistaken (at this time in Rome's history) Britain was regarded by the Romans as a mysterious, unknown land, the "edge of the world" so to speak, beyond which they had no idea what existed other than ocean. How would they have communicated? Would Caesar's Gallic horsemen have been able to talk to them i.e would there have been similarities in their tribal languages the way there are similarities between Spanish and Portuguese? Does Caesar even address this in his commentaries?
Caesar very rarely mentions the nuts and bolts of interpreters and the like. In Book 1 of Gallic Wars, he describes a scene in which he dismisses the "normal interpreters" (cotidianis interpretibus remotis) in order to speak with the Gallic chief Divitiacus directly. It is clear that he and the Gaul can't speak directly, not even in Greek, because Caesar uses a certain Gaius Valerius Troucillus/Procillus as a private interpreter. This suggests he didn't necessarily trust the "regular" interpreters that much. This Valerius was a Gaul himself, but also a second generation Roman citizen. He spoke Gallic (obvious) and either Latin or Greek (unclear which); he may even have spoken some Germanic, since he goes as an envoy to Ariovistus later.
I'm not a Celtic linguistic expert, but the basic understanding of Celtic language in this time period (mid-1st BCE) is that Gallic of the continent and Brittonic formed a shared "branch" of Celtic, distinct from Celto-Iberian. Gallic and Brittonic weren't the same language, or even dialects of the same language, but they may have been close enough to be mutually intelligible on a basic level. Certainly a guy like Valerius, with enough knowledge of different regional languages to translate back and forth, would have been sufficient for Caesar's aims in Britain. In book 4 he mentions "merchants" who ply the waters between the continent and SE Britain, and they must have been able to operate freely between those two worlds as far as basic communication. According to Caesar, some of them quickly went to Britain to alert them that the Romans were preparing to cross.