Google can now crawl and index "locale-adaptive" web pages. More details about the locale-aware crawl configurations of Googlebot can be found on the Google Support's website.
A page is "locale-adaptive" when the content that is being served is adapted based on the Accept-Language HTTP request header or the perceived location (IP address).
This tool allows you to quickly see if a page is locale-adaptive.
Learn more about international SEO
hreflang tags help indicate Google and Yandex in what language the content of a page is written and what audience (region) this content is targeting. This allows those search engines to serve the right content to the right user.
hreflang annotations can be hard to implement properly. This tool allows you to quickly check if hreflang tags on a page are correct.