Hi,
It used to be possible to get API responses from different locales with a single pair of public and private keys a few years ago but recently the Amazon API seems to have changed their policy not to.
When you register for Product Advertising API in your locale, use the locale-specific endpoint for each request (for example, https://webservices.amazon.com/onca/xml is the endpoint for the US).
You can send requests to the locale where you registered as a Product Advertising API developer. To use other endpoints, you must register as a Product Advertising API developer for that locale.
(Amazon Product Advertising API Docs)
According to the API documentation, it seems you can issue API keys for a different locale from the Amazon Associates’ web site that corresponds to the locale you want. (https://docs.aws.amazon.com/AWSECommerceService/latest/DG/becomingDev.html).