fix: wrong currency code returned from fiatCurrencyString

[Syn-McJ]

Issue being fixed or feature implemented

When fiatCurrencyString is called with a custom currency code, the returned amount is calculated correctly but the currency code is wrong - it is set to the local currency code.

What was done?

To format the currency correctly, the localFormat.currencyCode is changed before formatting and then restored to the original value.

How Has This Been Tested?

manually

Breaking Changes

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone