In the eastern provinces, where knowledge of Greek was widespread, Roman generals and their staffs (who, as members of the elite, were usually conversant in Greek) had no problems. In the west, the Romans typically relied on bilingual locals.

During his conquest of Gaul, Caesar seems to have had a staff of interpreters (probably from Cisalpine Gaul), whom he supplemented with Romanized local notables:

"Therefore, before attempting anything in the matter, Caesar ordered Diviciacus to be summoned to his quarters, and, having removed the regular interpreters, conversed with him through the mouth of Gaius Valerius Procillus, a leading man in the Province of Gaul and his own intimate friend, in whom he had the utmost confidence upon all matters." (BG 1.19)

Procillus (who may have been the son of a Gallic chieftain) is representative of the class of men who tended to spring up on the margins of the Roman Empire. A complex array of sociopolitical motives encouraged local potentates on both sides of the border to learn Latin, or have their sons learn it. Sometimes, the process was actively encouraged by Rome, most famously by the general Agricola in Britain:

"[Agricola] likewise provided a liberal education for the sons of the chiefs, and showed such a preference for the natural powers of the Britons over the industry of the Gauls that they who lately disdained the tongue of Rome now coveted its eloquence. " (Tacitus, Agricola 21)

(Tacitus' reference to the "industry of Gauls" refers to the schools of Latin rhetoric already widespread in Gaul a century after Caesar's conquest. )

More generally, however, locals learned Latin on their own initiative, even beyond the Roman border. In the imperial era, the existence of large permanent garrisons on the frontiers created a thriving economic zone that drew in local populations on both sides of the border - and encouraged them to learn Latin, if only to profit from the legions. Extensive recruitment of auxiliaries from beyond the frontiers, likewise, spread a working knowledge of Latin far beyond the border zone.

When the Romans expanded into new territory, in short, they usually found locals fluent in their language already there.