Geoffrey

To improve your SEO and the accessibility of your content, you must declare changes of language within your content. 
You must also declare translations of your pages to Google to avoid duplicate content.
TinyMCE: For the people still in this old editor, buttons are available to add <code>lang</code> and <code>hreflang</code> attributes on your content, therefore to improve accessibility when screen readers are reading words from different languages than your main page language.
Gutenberg: For Gutenberg users, WordPress already provide the “Language” menu. When selecting a text you can add a specific language. This plugin will enhance this behaviour with its own command, and by remembering the last language code you used. 
This plugin also comes with a hreflang option that you can put on links to tell users “this link lead to a French website” for example.
Example of cases you need this plugin:
<ul>
<li>You translated a blog post in your language from another blog, (hreflang alternate links)</li>
<li>You publish the same article in 2 languages on 2 different websites, (hreflang alternate links)</li>
<li>You use in your content a jargon from another language. (<code>lang</code> attribute)</li>
<li>You link from your post other posts in another language. (<code>hreflang</code> attribute)</li>
<li>Your Menus have some items not translated (<code>lang</code> attributes) or lead to page in another language (<code>hreflang</code>), you can edit those attributes in the Menus admin-menu. (WordPress 5.4 compatibility) If you have a FSE (Full Site Editing) theme, I don’t know how this will work for you, but you can use the Gutenberg commands this plugin put at your disposal.</li>
</ul>
<h3>Features available</h3>
<ul>
<li>Custom <code>hreflang</code> alternate links for posts (page, and custom posts)</li>
<li>Attributes <code>hreflang</code> and <code>lang</code> available and visible in the editor</li>
<li>Attributes <code>hreflang</code> and <code>lang</code> available on your main Menus items. (WordPress 5.4 compatibility)</li>
</ul>
<h3>Known bugs</h3>
In old version of Gutenberg, the <code>hreflang</code> attribute on links is applied, but never save it in database. That is because <code>hreflang</code> is cleaned from WordPress posts. Indeed, it was not well recognized by the WordPress post sanitizing function.
You can donate to support
<ul>
<li><a href="https://www.paypal.com/cgi-bin/webscr?cmd=_donations&business=P39NJPCWVXGDY&lc=FR&item_name=Juiz%20Lang%20Attributes2d%20WP%20Plugin&item_number=%23wp%2djla&currency_code=EUR&bn=PP%2dDonationsBF%3abtn_donate_SM%2egif%3aNonHosted" rel="nofollow ugc">Donate what you want with Paypal</a></li>
</ul>
Please, use the support forum to tell me bugs encountered, and be patient
<h3>Other plugins</h3>
<a href="https://wordpress.org/plugins/juiz-social-post-sharer/" rel="ugc">Juiz Social Post Sharer</a>: if you need social buttons. 
<a href="https://fr.wordpress.org/plugins/juiz-outdated-post-message/" rel="nofollow ugc">Juiz Outdated Post Message</a>: if you want to put a warning on old post on your blog. 
<a href="https://wordpress.org/plugins/embed-can-i-use/" rel="ugc">Embed Can I Use</a>: if you talk about support of HTML 5, CSS3 or JS API, you need to embed Can I Use support tables. 
<a href="https://wordpress.org/plugins/social-integration-for-bluesky/" rel="ugc">Social integration for BlueSky</a>: Add a Profile Banner, a list of your recent BlueSky Posts, or an auto syndication on BlueSky with this plugin.

Juiz Lang Attribute

Add a custom HREFLANG meta box on your post to manually edit the link between your post and a translation (which could be outside your domain).

Marco Giannini

Just install this plugin for print:
link rel=”alternate” hreflang=”the language” href=”the url”
This plugin needs Polylang for working. 
No WPML supports.
This plugin needs Polylang to be activated for working.
This plugin auto prints all the alternate language of the current page. If no alternate language found it prints the homepage alternate URL.

Hreflang for polylang

Auto print in header the link alternate hreflang of all the language in the website.

Juiz Lang Attribute vs Hreflang for polylang