The English word "wallet" (as in something you carry in your pocket to hold money, credit cards, etc.) can be translated into Spanish as cartera or billetera. Are the words synonyms that can be used interchangeably, or does each country generally use one word or the other? If it is a regional difference, what areas of the Spanish-speaking world generally use each word?
It is a regional choice, as with many other nouns.
In Chile, billetera is the only translation of wallet (though we would understand the use of cartera from a foreign person), but cartera is actually purse, as in handbag: a bag a woman uses to carry stuff, which is usually much bigger than a wallet and does not fit in a pocket.
In other countries, words like bolsa and bolso are used for purse, while cartera is used for wallet.
My guess is that cartera is more widespread, because it is also the origin of carterista (pickpocket), which is more standard. If you intend to translate the word for a global audience, you should go for cartera.
In Colombia, the word
cartera is mostly used to refer to a woman's Bolso (handbag).
I should mention that
cartera is falling out of use in Colombia. I can only remember old people using that word.
Bolso is the preferred word nowadays.
I am almost certain that in other Latin American countries (Argentina comes to mind) the words
billetera mean the same; in other words, a man can say:
Se me olvidó mi cartera but you wouldn't hear that coming from a male in Colombia.