First, I would create string resources for each of the shipping method names (this way you can create different method names for each locale).
The right way to continue would be to open up the RTShipping class in the AspDotNetStorefrontCore project (requires source code), and implement your own check of the methods returned, and based on the method returned instead of using the name returned by USPS or FedEx replace it with the appropriate string resource for the customers current locale. This method requires a recompile.
If you don't have the source code, you could a similar thing in the checkoutshipping.aspx.cs page, but you'd have to resort to creating a method that would do a String.Replace on the shipping method HTML that is rendered to the page, looking for each instance of shipping methods you know will be returned by FedEx and USPS and replacing them with the appropriate string resource. This method could get a bit messy, but should still get the job done without requiring source code.
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