I'm not really sure why there would be much difficulty in communication. The Gauls had had close contact with the Romans for centuries before Caesar showed up in 58, and the Cisalpines were either full citizens (the Cispadanes) or held Latin status (the Transpadanes). Many of the Gallic peoples were allies of Caesar's, to say nothing of the Fifth Legion, which was composed of Transalpines who were granted citizenship. The Celtiberians had been subject to Roman provincial rule since the Punic Wars. Within Transalpine Gaul the Romans had held the province of Gallia Narbonensis since 125. The Transalpines also were apparently fairly homogeneous in terms of their language. Caesar mentions that the people of different parts of Gaul differed in their language but does not specify how. Strabo claimed that with the exception of the Aquitani the different languages that the Gauls spoke were all slight variations of the same language--clearly there was not much difficulty in communicating across dialects or Transalpine languages or whatever we want to classify this as. The Gauls also had had contact with the Greeks for centuries, including extremely close contact at places like Massilia. Greek influence among the Gauls was such that the OCD mentions that following the settlement of Massilia "in Gaul and eastern Spain the Greek presence had profound effects," mentioning imports of large quantities of Greek pottery up the Rhone and along the Iberian and Gallic coasts, as well as influence in fortifications and the construction of the Gallic oppida and the introduction of wine and the olive. Some Gallic towns, such as Glanum, were wholly taken over by Greeks, and Massilia entered into amicitia with the Romans from an early date, assisting them against Hannibal--Massilian request for Roman aid against Gallic attacks in 125 resulted in the formation of Gallia Narbonensis. Greek influence was great enough that Caesar says that "publicis privatisque rationibus Graecis litteris utantur" (in public and private matters they use Greek letters). Caesar means the Greek alphabet, not their language, but in any case the Gallic elite must have been intimately familiar with both Greek and Latin and we must imagine no difficulty in communicating with them. With the Britons there was naturally less contact. I don't know how similar British languages were to Gallic (though I'd be surprised if they differed enough to present too much of a problem, given the close ties between the Britons and the Belgae) but there should have been no difficulty with Gallic leaders interpreting--Caesar never mentions a difficulty with communication