The main purpose of using hreflang is to guide users to a linguistically-appropriate version of your website. However, even with a large selection of languages, it is also possible that users cannot be assigned a specific available variant due to language or country codes. If these users encounter your website via a search engine, the ranking decides which of the language variants are displayed. In this case, you may end up losing potential readers after just a few seconds, when they land on the wrong language version. However, with ‘x-default’, Google provides an option which you can use to diffuse the situation.
The value you can set as an alternative to the ISO codes lets the search engine know that the linked URL is the default option for all users who do not have an explicitly named language version. It would then be possible to present a language overview on this default page, with a selection option so that the visitor can select the language themselves. To do this, you will need to add the appropriate code line to the alternative version’s header, which is as follows: