Is AppleCare+ valid in other countries?

Yes, but with certain limitations ...

Taken directly from: Legal - AppleCare+ - Apple

Service will be limited to the options available in the country where you request service. Service options, parts availability and response times may vary. If service is not available for the Covered Equipment in a country that is not the country of purchase, you may be responsible for shipping and handling charges to facilitate service in a country where service is available. If you seek service in a country that is not the country of purchase, you must comply with all applicable import and export laws and regulations and be responsible for all custom duties, V.A.T. and other associated sales taxes and charges. For international service, Apple may repair or exchange products and parts with comparable products and parts that comply with local standards.

AppleCare+ Coverage depends on the region it was purchased in and the device it was purchased for. I know that AppleCare+ in most countries is only available in the country of purchase, but check the terms and conditions for your specific product and location.