johanee

On every pageload a localized WordPress site spends a huge amount of time setting up the translation (l10n) object. Startup for a localized site can be almost four times slower than an untranslated one.
This plugin stores the translation object between pageloads so it only has to be created once, resulting in significantly better performance (performance graph included with screenshot).
Plugin supports caching multiple languages (if you use a multi-language plugin).
Available storage: Plain file, APC (Alternative PHP Cache), or SHMOP (Shared Memory)

Cache Translation Object

Substantially increase performance of localized WordPress by caching the translation (l10n) object. Supports multiple languages.

docjojo

atec Translation Status is a diagnostic and maintenance plugin that helps you understand the real state of translations in WordPress.
Modern WordPress automatically compiles translation files for performance. When translations are missing, outdated, or incorrectly loaded, WordPress usually fails silently — no warnings, no errors. 
This plugin makes translation issues visible and actionable.
What it does
<ul>
<li>Lists all active (loaded) textdomains</li>
<li>Detects whether translation files (.mo) exist</li>
<li>Detects whether compiled translation files (.l10n.php) exist (WP 6.5+)</li>
<li>Checks whether compiled files are out of date</li>
<li>Clearly indicates when:
<ul>
<li>translations are missing</li>
<li>compiled files need rebuilding</li>
<li>plugins or themes do not follow WordPress translation conventions</li>
</ul>
</li>
</ul>
What it does not do
<ul>
<li>It does not replace WordPress’ translation system</li>
<li>It does not generate translations</li>
<li>It does not modify plugin or theme code</li>
</ul>
Instead, it helps you identify problems, rebuild safely, or contact the responsible plugin/theme developer with clear evidence.
<h3>Third-Party Services</h3>
<h3>Integrity check</h3>
Once, when activating the plugin, an integrity check is requested from our server – if you give your permission. 
Source: https://atecplugins.com/ 
Privacy policy: https://atecplugins.com/privacy-policy/

atec Translation Status

Check, diagnose, and validate WordPress core, plugin, and theme translations — including compiled translation status (WP 6.5+).

Cache Translation Object vs atec Translation Status