I’m not sure I’m following, sorry. For example, if your Square merchant account is based in the US, then you can only accept USD. This means, that even if a customer from the GB comes to your site, they will be charged in USD via the API. On their bank’s side, the bank will automatically convert the USD to GBD to see how much to actually take out and charge them. This is not something the Square APIs will do, but something the bank will do.
As for your last question, you can find the currency using the ListMerchants endpoint.