Can my portal show prices in my customers' own currency and in their own language?

Yes, a white label portal can normally show prices in more than one currency and text in more than one language, but how far that goes depends on your plan and on what each supplier sends back.

With currency, some suppliers return a rate only in their own currency, so the portal converts it for display using a rate you control. Others allow the booking to be made in the currency you ask for. That difference matters at payment, because the amount your customer is actually charged has to be one your payment gateway and your supplier can both settle. Where a rate is converted for display only, it is fair to tell the customer the final charge may vary a little with the rate on the day.

Language works differently. The wording that belongs to your portal, such as the buttons, the search labels, the emails and the vouchers, can be translated and kept as your own text. The content that comes from a supplier, such as a hotel description, a room name or a fare rule, arrives in whatever language that supplier provides, and very often that is English only. So your portal can feel local to your customer while a few supplier lines stay in English.

The sensible next step is to check which currencies your payment gateway can accept and settle in, and ask your suppliers which of them can book in a currency other than their default, because those two answers decide what is realistic for you.