Steph

This plugin assumes you have a site in two or more languages, and that you are using a separate WordPress install for each language. It helps you create links between the same pages in different 
language versions.
You should have a pattern of parallel sites with urls like http://stephanie-booth.com/en/, http://stephanie-booth.com/de/, http://stephanie-booth.com/fr/. Maybe the page 
http://stephanie-booth.com/en/about is equivalent to the page http://stephanie-booth.com/fr/a-propos. The plugin adds a DBX box in the edit/create page admin form, named “Other Language Slugs”. In 
this case, you’d edit the “about” page, and paste the fr slug (“a-propos”) in the appropriate field.
The plugin will then display a small link to the page in the other language at the bottom of your page, or in your menu bar if you’re using a Sandbox-based theme.
Careful! This only works for Pages, not posts!
For intelligent user redirection when they land on the home page, you might want to try using the technique described here: http://climbtothestars.org/archives/2007/12/28/browser-language-detection-and-redirection/
<h3>Future Development</h3>
Here’s what I’d like this plugin to do, someday:
<ul>
<li>automatically capture URLs like http://stephanie-booth.com/en/a-propos and redirect them to http://stephanie-booth.com/en/about (that’s what happens when people edit URLs directly, just replacing 
the language code — I do it all the time)</li>
</ul>

Bunny’s Language Linker

Helps you create links between equivalent (but different language) pages between WordPress installations.

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.

Bunny’s Language Linker vs Preferred Languages