Looking at ancient coins, I see many struck by Celtic tribes of Gaul, with inscriptions made up of Latin letters:
What no one ever says is what those inscriptions are supposed to mean. Is this known? Why were they using Latin letters anyway? If they were subdued by Rome then wouldn't the inscriptions be a lot more familiar, and if they were not subdued then why use Latin letters at all?
In many places, notably the Arabian Pennisula, when there were not enough coins in circulation rulers would have more coins struck. However, instead of make coins with some other design local rulers would try, to a varying degree of success to imitate coins that were made by influential, neighboring powers.
Gaul became another hotbed of imitators. Just because the coins were imitations does not mean it was worthless, the coins would still be X amount of silver. Now to answer your questions.
Most people did not take coins out of their lack of pockets ands read them. People could get the gist of coins of by seeing emperor X's head and seeing the military unit or god on the back. Despite not knowing the language the coinage and the designs were familiar and recognizable. And even if they weren't able to understand the coin merchants would still accept it because x amount of silver was still worth x amount of silver.
Sources:
Dacian Copies & Imitations by Philip Davis
The Tale of Thurrock Potins by Mark Fox
http://worldcoincatalog.com/AC/C1/CelticKingdoms/CelticKingdoms.htm