I know some tribes joined the Romans. But how would these agreements have been made without a knowledge of the language previous to these meetings. And if they had a translator, where would they have found someone with knowledge of both Latin and the relevant Brittonic language.
This isn't a particularly interesting answer, but Julius Caesar had a large number of Gallic allies with him. Rome had an influence in the region for quite some time so it is certain that, at least with the larger kingdoms such as the Aedui, there would be many who could speak Latin. So there is a chain he could follow. But perhaps more importantly he had druids among his allies, and as they were religious specialists they would have had a wide range of connections. They seem to have valued Britain in particular as an origin of their religious practice, so it is likely they could communicated with some Britons.