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This extension has been inspired by http://devel.kostdoktorn.se/cache-translation-object and gives up to 5x speed increase when using WordPress with “custom” (i.e. non English) localisation files.
MO object created by WordPress is cached and stored in the <code>/wp-content/cache/</code> directory and then used when loading localisations.

l10n cache

Improves site performance by providing caching for localisation objects (*.mo files).

Pascal Birchler

Thanks to language packs it’s easier than ever before to change the main language of your site. 
However, in some cases a single locale is not enough. When WordPress can’t find a translation for the active locale, it falls back to the original English strings. 
That’s a poor user experience for many non-English speakers.
This feature project aims to change that by letting users choose multiple languages for displaying WordPress in. 
That way you can set some sort of “fallback chain” where WordPress tries to load translations in your preferred order.
Please help us test this plugin and let us know if something is not working as you think it should.
Keyboard Shortcuts
<ul>
<li><code>Arrow Up</code>: Move selected locale one position up.</li>
<li><code>Arrow Down</code>: Move selected locale one position down.</li>
<li><code>Home</code>: Select first locale in the list.</li>
<li><code>End</code>: Select last locale in the list.</li>
<li><code>Backspace</code>/<code>Delete</code>: remove the selected locale from the list.</li>
<li><code>Alt+A</code>: Add the current locale from the dropdown to the list.</li>
</ul>
Note: the Preferred Languages UI needs to be focused in order for the keyboard shortcuts to work.
Merging Translations
Previously, only the first available translation for a given locale and domain will be loaded. 
However, when translations are incomplete, some strings might still be displayed in English. 
That’s a poor user experience as well.
To prevent this, Preferred Languages now automatically merges all incomplete translations in the list.
the <code>preferred_languages_merge_translations</code> filter can be used to opt out of this behavior. 
It provides three parameters:
<ol>
<li><code>$merge</code> – Whether translations should be merged. Defaults to <code>true</code>.</li>
<li><code>$domain</code> – The text domain</li>
<li><code>$current_locale</code> – The current locale.</li>
</ol>
<h4>Get Involved</h4>
Active development is taking place on <a href="https://github.com/swissspidy/preferred-languages" rel="nofollow ugc">GitHub</a>.
If you want to get involved, check out <a href="https://github.com/swissspidy/preferred-languages/issues" rel="nofollow ugc">open issues</a> and join the <a href="https://wordpress.slack.com/messages/core-i18n" rel="nofollow ugc">#core-i18n</a> channel on <a href="https://wordpress.slack.com/" rel="nofollow ugc">Slack</a>. If you don’t have a Slack account yet, you can sign up at <a href="https://make.wordpress.org/chat/" rel="nofollow ugc">make.wordpress.org/chat/</a>.

Preferred Languages

Esto es imprescindible para cualquier usuario no angloparlante. El estado de las traducciones es irregular y sería perfecto tener un segundo e incluso un tercer idioma local para cargar si algo falta en tu idioma local, y así que no salga media web en español - por ejemplo - y el resto en inglés.

Esto ya estaría en WordPress si los principales equipos de desarrollo no fuesen angloparlantes, y claro, para ellos no es una prioridad ni de lejos.

Imprescindible - Debería estar ya en WordPress

Thanks for Pascal to solve the problem of fallback languages. It is time to add it to the core and would show that WordPress is thinking globally.

In core, please

I think this should be in Core for websites with several possible languages. I like it very much! Specially I appreciate that each user can also choose languages from the selected languages. 
(I am developing a translation plugin for my own needs to use Google Translate to translate posts/pages and complement it.)

Very necessary in core


I honestly think that this should be a core feature of WP. 



The only thing wrong I notice is that when you are looking at the front end... The language of the admin bar changes into the site language but that's the only issue I have with this plugin at the time of writing. For the rest, it's a dream plugin.


Make this a core feature

Should be in the core!

Choose languages for displaying WordPress in, in order of preference.

l10n cache vs Preferred Languages