Question about Latin and Trade

Hi everyone!

I have a Latin-related question that has been buggering me for a long time now, maybe some of you could help me out?
(And yes, I have tried google many MANY times, but haven't come up with an answer yet.. :-/ )

This is the question; 

I know many traders learned the language by selling their wares to the roman military, but I’m guessing there were a lot more possibilities for traders to learn Latin. Also, how did the language spread from those traders to other non-latin speaking communities?

Could you help me out please? 
(Or help me find some usefull links?)
Thanks!

well first you have to define the period at which you're looking, and then the region as the trade dynamics were not the same everywhere. 

For instance in the Republican period we have roman traders going all around Gaul, They would teach some latin, or use translators that might teach their skill to others. But we know from Caesar that there were more chances that the Gauls might now the greek alphabet (see the Helvetii's tablets after their defeat). 

In the imperial period you had both traders going out of the empire into the wilderness, and traders from barbarian lands comming to outposts to sell their wares. The roman traders would go to villages/town and then bargain, either with the help of a translator or with people who, by their elite statute might already know latin thanks to previous association with the empire (Arminius had been in the Roman army for instance, so he spoke latin). Others might then want to learn the langage to be able to show they too can be part of this elite. Soldiers might also become traders themselve after their time serving the army, their "pension bonus" providing them the cash to establish their business and their former comrades becoming their new clients. 

Finally the foreign traders might also learn latin in inns and taverns inside roman territory...

Also remember that the langage situation could be hyper-local : I remember visiting two cities on the Danube, some 50km from each other, one with all its epigraphy in latin and the other in greek... 

The demand of the Roman world for people to speak latin only really impacts upon administration and politics, where we see the local Italian languages become recessive and maternal in persistence whereas latin dominates the paternal line because of the need to earn a living in a more expansive state, instead of the tribal regions cooperating but not actually run from Rome. I don't dispute that latin spread through trading, but learning the native language, or adopting a hybrid slang, would ordinarily be more conducive to getting a good deal.