The hreflang attribute is an HTML attribute used to specify the language and optionally the geographic targeting of a webpage. It is primarily used for websites that offer content in multiple languages or target different regions with the same language. 
Here’s a breakdown of its purpose and usage:
Purpose:
  • Informs Search Engines: The hreflang attribute tells search engines like Google which language and regional version of a page should be displayed to users based on their language preferences and location.
  • Improves User Experience: By serving the correct language version, it ensures users are presented with content they can understand, enhancing their experience and potentially reducing bounce rates.
  • International SEO: It plays a vital role in international SEO by helping search engines understand the relationship between different language versions of a website, avoiding duplicate content issues, and ensuring the right version ranks for the right audience. 
How it works:
  • Language and Region Codes: The hreflang attribute uses language codes (ISO 639-1 format) and optionally region codes (ISO 3166-1 Alpha 2 format) to specify the target audience. For instance, hreflang="en-US" indicates English language content for the United States.
  • Bidirectional Linking: Each language version should include a hreflang attribute referencing itself and all other language versions, creating bidirectional links so search engines can correctly understand the relationships.
  • x-default: This special value is used to specify a default page for users whose language or region doesn’t match any of the explicitly declared hreflang values, acting as a fallback. 
Implementation Methods:
  • HTML <link> element: The most common method involves adding <link rel="alternate" hreflang="..."> tags within the <head> section of the HTML document.
  • HTTP header: Used for non-HTML resources like PDFs, indicating alternative language versions through the HTTP header.
  • Sitemap: Alternative language versions can also be specified within XML sitemaps. 
Importance:
  • Avoiding Duplicate Content: Helps prevent search engines from mistakenly treating different language versions as duplicate content, which can negatively affect ranking.
  • Targeting the Right Audience: Ensures the most relevant version of a page is displayed to users based on their language and region.
  • Improved Search Visibility: Increases the chances of ranking higher in search results within specific language and regional markets. 
In summary, the hreflang attribute is a valuable tool for international SEO, enabling websites to effectively target diverse audiences with the most relevant content by clearly communicating language and regional variations to search engines.