El Salvador has now passed a law that makes bitcoin the country's legal tender. Does this mean that when the law comes into effect in September 2021, other countries will presumably start treating bitcoin as a foreign currency?
It is possible that few countries consider bitcoin as foreign currency. Every country has their own laws for foreign exchange.
Example: Foreign Exchange Management Act, 1999 (FEMA) in India has below definitions related to currencies
'Foreign currency' means any currency other than Indian currency
Foreign Currency Account means an account held or maintained in currency other than the currency of India or Nepal or Bhutan
In pursuance of clause (h) of Section 2 of the Foreign Exchange Management Act, 1999 (42 of 1999), the Reserve Bank notifies debit cards, ATM cards or any other instrument by whatever name called that can be used to create a financial liability, as ‘currency’.
'Permitted currency' means a foreign currency which is freely convertible