In processing a document with a new supplier to Sage 50, you can chose whether to set this up in your account currency or multiple other currencies.
If the 'Publish multiple currencies' toggle is turned off (default), then the suppliers and items will be in your account currency.
To create suppliers and send items in other currencies, the 'Publish multiple currencies' toggle must be on in Dext Prepare and the 'Foreign Trader Module' is turned on in Sage. To switch the Foreign Trader Module on within Sage50, you can find instructions to do this on the relevant Sage article.
The 'Publish multiple currencies' toggle can be found by navigating to
Manage > Connections > Integrations