Create the hreflang link attribute markup for your multilingual website, keeping in mind the bidirectional linking of content. All equivalent content has to be linked to each other or it may happen that Google does not consider the markup valid.
The hreflang-Guide for international SEO
Here you will learn everything about the correct handling of Google and multilingual websites. Avoid duplicate content and learn how to use the hreflang link attribute.
Where and how do I implement the hreflang remark?
Google recommends using the hreflang annotation in the following scenarios:
<link rel="alternate" hreflang="en-ca" href="http://www.domain.com/ca/" />
The language of the document is English (en) and it is intended for the region of Canda (ca). Consequently, the content at www.domain.com/ca/ is marked for English speaking customers in Canada.
The respective language (en) must always be specified in ISO 639-1 format. The country code (-CA) is optional and must be specified in ISO 3166-1 Alpha 2 format.
Nice is, however, to stick to the convention and write the ISO code of the language in lower case and the optional ISO code of the region in upper case.
How do I use the hreflang annotation correctly?
When using the hreflang annotation it is important not to forget the bidirectional linking. This means that all equivalent contents must be linked to each other. This is the only way Google can understand the network architecture of the website. If the hreflang annotation is only placed ?in one direction? the structure is broken and from Google’s point of view not valid.