Do you have to declare interest earned with USA tax filing if you earned this in a foreign country and already paid taxes in that country?

Yes, if you are U.S. citizen or resident.
You will also claim credit for taxes paid in the foreign country. For this you will file Form 1116.

And if the foreign bank account value exceeds $10,000 at any point during the year, you need to file Form TD F 90-22.1.