If you can write your posts in 2 or more(Woo…) languages within one blog, you might want to tell the client browser the language attribute of you posts.
According to <a href="http://www.w3.org/International/tutorials/language-decl/en/all.html" title=" W3C tutorial" rel="nofollow ugc">the tutorial on W3C</a>, language attribute can be declared in the container html tag of your post like this: 
<div id=”post_id” class=”post_entry” lang=”zh-Hant” xml:lang=”zh-Hant” dir=”ttb”>… 
Here “zh-Hans” is the language attribute of your post which means it’s written in Simplified Chinese.
This plugin gives your post a language tag by adding a meta deta to that post.
To add language tag, select it from the language box when you compose/edit a post. Select “None” will delete the language tag.
<h3>Revisions</h3>
2007-12-18, 
0.2: Now you can difine language direction. 
 Insert CSS style into page head automatically to support pretty printing.
 readme file updated for new feature and fixed a error
2007-07-23 
0.1: First release

Multilingual post

This plugin adds a standard compliant language tag to your post and page.

Chouby

erichie

ipoulsen

mbrown97

Hector Lopez

Robert Hanna

penyaskito

smithworx

elliothanna

Ray Enterprise Translation offers convenient cloud-based localization and translation for WordPress by simplifying the process of creating and maintaining your multilingual website. Features include machine, professional, and community translation solutions that ease the manual burden of launching in new markets while improving language consistency, cutting costs, and enabling you to deliver your brand to consumers abroad in the quickest and most effective way possible.
Make your WordPress website content truly multilingual. The power to translate is now inside WordPress!
<h4>Customizable Translation Workflows</h4>
WordPress administrators use Translation Profiles to categorize content by its relative value. You associate each content type with a Translation Profile, and the plugin carries out the appropriate workflow:
<ul>
<li>
Professional – The plugin enables you to use your own translation agency or tap into Ray Enterprise’s marketplace of more than 5,000+ in-country translators. Don’t have time to manage your own translation project? Let Ray Enterprise take care of it for you. The plugin fully automates file transfer between WordPress and Ray Enterprise. You’ll have full visibility into the translation process every step of the way. And once the translations are completed, they’ll automatically download and publish to your website according to the preferences you’ve set.
</li>
<li>
Community – If you’re looking to save money by avoiding professional translation, you can take the do-it-yourself approach and have your bilingual employees, partners and/or users perform translations right within WordPress. The plugin integrates with and provides use of the Ray Enterprise Workbench, a professional-grade text editor used for translating, reviewing, and post-editing multilingual content.
</li>
<li>
Free Automatic – Machine translation is an excellent option if you’re on a tight budget, looking for near-instant results, and are okay with less-than-perfect quality. The plugin allows you to automatically translate your site by providing the commercial API for Microsoft Translator. You can upload up to 20,000 processed words in the Ray Enterprise system for free (we calculate processed words by multiplying the source word count by the number of requested translations). Machine translations can be post-edited at any time using the Ray Enterprise Workbench.
</li>
</ul>
<h4>Cloud-Based Translation Management System</h4>
Need access to an enterprise level Translation Management System designed to work directly with WordPress and other content management systems, allowing you complete transparent access with all your translation projects? The cloud-based Ray Enterprise Translation Management Systems helps your business access new markets and customers. Contact sales@lingotek.com for more information and pricing of these features.
<h4>Benefits</h4>
<ul>
<li>Cost Savings</li>
<li>Faster Translation Times</li>
<li>Higher Quality</li>
<li>Real-time Monitoring</li>
<li>Eliminate the Copy/Paste Method of Translating</li>
<li>Increased Involvement and Loyalty</li>
<li>In-context Review</li>
</ul>
<h4>Features</h4>
Ray Enterprise works in conjuction with the <a href="https://wordpress.org/plugins/polylang/" rel="ugc">Polylang</a> plugin (the plumbing to make WordPress multilingual ready) allowing you to create a bilingual or multilingual WordPress site. You write posts, pages, and create categories and post tags as usual, and then define the language for each of them.
<ul>
<li>You can use as many languages as you want. RTL language scripts are supported. WordPress languages packs are automatically downloaded and updated.</li>
<li>You can translate posts, pages, media, categories, post tags, menus, widgets…</li>
<li>Custom post types, custom taxonomies, sticky posts and post formats, RSS feeds and all default WordPress widgets are supported.</li>
<li>The language is either set by the content or by the language code in the url, or you can use one different subdomain or domain per language.</li>
<li>Categories, post tags, and some other metas are automatically copied when adding a new post or page translation.</li>
<li>A customizable language switcher is provided as a widget or in the nav menu.</li>
<li>The admin interface is of course multilingual too and each user can set the WordPress admin language in its profile.</li>
<li>We now support localization of pages created with Elementor</li>
<li>We maintain the privacy of free automatic users by deleting all completed content from our TMS older than one month and active content older than two months. To ensure this does not affect your translated content, please download your translation soon after they are complete, or set your download settings to automatic. Enterprise customers should coordinate with client success about their community’s archive and deletion timeframes.</li>
</ul>
<h4>Credits</h4>
Wherever third party code has been used, credit has been given in the code’s comments.
<h4>Do you like Ray Enterprise?</h4>
Don’t hesitate to <a href="https://wordpress.org/support/view/plugin-reviews/lingotek-translation" rel="ugc">give your feedback</a>. It will help make the plugin better. Other <a href="https://wordpress.org/support/plugin/lingotek-translation" rel="ugc">contributions</a> (helping other users on the support forum) are welcome!
<h3>Third-Party Services</h3>
This plugin relies on the following third-party services:
<ol>
<li>
Lingotek API
<ul>
<li>Description: This plugin uses the Lingotek API to manage translations.</li>
<li>Service URL: <a href="https://myaccount.lingotek.com" rel="nofollow ugc">https://myaccount.lingotek.com</a></li>
<li>Privacy Policy: <a href="https://www.lingotek.com/privacy-policy" rel="nofollow ugc">Lingotek Privacy Policy</a></li>
</ul>
</li>
<li>
Lingotek GMC
<ul>
<li>Description: This plugin uses Lingotek GMC for loading scripts and managing locales.</li>
<li>Service URL: <a href="https://gmc.lingotek.com" rel="nofollow ugc">https://gmc.lingotek.com</a></li>
<li>Privacy Policy: <a href="https://www.lingotek.com/privacy-policy" rel="nofollow ugc">Lingotek Privacy Policy</a></li>
</ul>
</li>
<li>
Lingotek Marketplace
<ul>
<li>Description: This plugin uses the Lingotek Marketplace API for transaction translations.</li>
<li>Service URL: <a href="https://marketplace.lingotek.com" rel="nofollow ugc">https://marketplace.lingotek.com</a></li>
<li>Privacy Policy: <a href="https://www.lingotek.com/privacy-policy" rel="nofollow ugc">Lingotek Privacy Policy</a></li>
</ul>
</li>
<li>
Lingotek Project
<ul>
<li>Description: This plugin uses the Lingotek project for content translation for WordPress CMS.</li>
<li>Service URL: <a href="https://lingotek.com" rel="nofollow ugc">https://lingotek.com</a></li>
<li>Privacy Policy: <a href="https://www.lingotek.com/privacy-policy" rel="nofollow ugc">Lingotek Privacy Policy</a></li>
</ul>
</li>
<li>
Straker Translations Insight
<ul>
<li>Description: This plugin uses Straker Translations Insight for freemium upgrade connectors.</li>
<li>Service URL: <a href="https://insight.strakertranslations.com/lingotek-freemium-upgrade-connectors" rel="nofollow ugc">https://insight.strakertranslations.com/lingotek-freemium-upgrade-connectors</a></li>
<li>Privacy Policy: <a href="https://www.strakertranslations.com/privacy-policy" rel="nofollow ugc">Straker Translations Privacy Policy</a></li>
</ul>
</li>
<li>
Polylang Plugin
<ul>
<li>Description: This plugin integrates with the Polylang plugin for multilingual support.</li>
<li>Service URL: <a href="https://wordpress.org/plugins/polylang/" rel="ugc">https://wordpress.org/plugins/polylang/</a></li>
<li>Privacy Policy: <a href="https://polylang.pro/privacy-policy/" rel="nofollow ugc">Polylang Privacy Policy</a></li>
</ul>
</li>
<li>
Gravatar
<ul>
<li>Description: This plugin uses Gravatar to display user avatars.</li>
<li>Service URL: <a href="https://www.gravatar.com" rel="nofollow ugc">https://www.gravatar.com</a></li>
<li>Privacy Policy: <a href="https://automattic.com/privacy/" rel="nofollow ugc">Gravatar Privacy Policy</a></li>
</ul>
</li>
</ol>
<h3>License</h3>
This plugin is licensed under the GNU General Public License v2.0. 
– License URL: <a href="https://www.gnu.org/licenses/gpl-2.0.html" rel="nofollow ugc">https://www.gnu.org/licenses/gpl-2.0.html</a>
<h3>Additional Information</h3>
bohemiancoding 
 – Description: This plugin includes SVG files that contain metadata related to the Sketch design tool: 
 – Sketch Metadata URL: <a href="http://www.bohemiancoding.com" rel="nofollow ugc">http://www.bohemiancoding.com</a>
W3C 
 – Description: This plugin references the W3C for SVG standards. 
 – Service URL: <a href="http://www.w3.org" rel="nofollow ugc">http://www.w3.org</a>
GitHub Repository 
 – Description: The source code for this plugin is hosted on GitHub. 
 – Repository URL: <a href="https://github.com/lingotek/lingotek-translation" rel="nofollow ugc">https://github.com/lingotek/lingotek-translation</a>
PHP Manual 
 – Description: This plugin references the PHP manual for the <code>include</code> function. 
 – Service URL: <a href="http://php.net/manual/en/function.include.php" rel="nofollow ugc">http://php.net/manual/en/function.include.php</a>
WordPress Profile 
 – Description: This plugin references the WordPress profile of a contributor. 
 – Service URL: <a href="https://profiles.wordpress.org/chouby" rel="nofollow ugc">https://profiles.wordpress.org/chouby</a>

Ray Enterprise Translation

it seems like the plugin is not maintained anymore, which is very unfortunate.
Currently on activation I saw many things not working as expected.

The first problem I encountered was that I wasn't able to connect my Lingotek account while the plugin Rank Math SEO Plugin was active. I checked the logs to see whats happening under the hood (nothing specific to RM but some warnings due to ternary operators which will cause fatal errors in PHP 8).

The UI is not very intuitive and feels like not been touched for years.
I am also not that happy with the machine translation (it's like google translate in 2010).

But of course community and professional translations can help translating pages faster so the plugin is not completly useless.

It just needs a major update!

needs a major update

Waste of time

Convenient cloud-based localization and translation for WordPress.

Multilingual post vs Ray Enterprise Translation