You can add hreflang tag easily by using this plugin https://wordpress.org/plugins/language-selector-related/
see the example..visit this site http://www.viduweb.com.au/about-us/
and check source code
<link rel=”alternate” href=”http://www.viduweb.com.au/about-us/” hreflang=”en-au” />
<link rel=”alternate” href=”http://www.viduweb.com/about-us/” hreflang=”x-default” />
Hello you can try this plugin for this. May be its helpful to you.
https://wordpress.org/plugins/language-selector-related/
Example…
<link rel=”alternate” href=”http://www.viduweb.com.au/about-us/” hreflang=”en-au” />
<link rel=”alternate” href=”http://www.viduweb.com/about-us/” hreflang=”x-default” />
