Since 2009, Xili-language followed two years later by Polylang are two plugins that use a taxonomy named “language”. This is possible since WP 2.3 – see file taxonomy.php
The main advantage of this approach taxonomy is that it does not destroy records (post) originals. (Indeed, other extensions modify the fields ‘post_content’ and add tables …)

Like those basic categories (category) and keywords / tags (post_tag), each element (term) of the taxonomy is described by its name, its shortcut (slug) and description. The other fields are the technical elements (term_id, taxonomy_id …)

Example of category “Recent News”
– Name : Recent News
– Slug : recent-news
– Description : the most recent news to be published

Inside xili-language and the taxinomy “language”, a language is defined as:
– Name : fr_FR (iso)
– Slug : fr_fr
– Description : French
and
with Polylang, the choice is different :
– Name : French
– Slug : fr (raccourci)
– Description : several serialized data in a table (array) whose name iso (locale => en_US) used by WP to name translation files.

The key point (pivot) is in both cases, iso code that describes the language and country of its use: fr_CA is used in Canada if we are to be precise.

Using serialized data in the description field is a Polylang originality that is not found in WP. Such fields do not allow SQL query. (need php functions)

As shown locale.php JetPack file (used by Xili-language), language management is very complex and use multiple encodings. You can find other invariant elements like the name cited in the language (French) and the writing direction (ltr). In fact, there is very little quantity of rtl languages.

With a few hundred lines, Xili-language (2.21+) can recover multilingual elements of a previous WP installation driven by Polylang. (see instructions post)
Xili-dictionary is also compatible with Polylang active (through filters and an abstraction of the taxonomy model). For example to translate language files for a theme or plugin with creation of pot, po, mo files.