Make get_available_languages() filtrable

[yoavf]

When a user object is saved using edit_user() (i.e, after editing the user settings) WP will discard (and replace with en_US) any submitted value for the new user locale meta if it is not part of the array returned by get_available_languages()

get_available_languages() works by scanning the language folder for .mo files. In some use cases, language files might not exist for a locale, named in a different way, or maybe stored elsewhere. get_available_languages() results should be filtrable.

FWIW: we need this filter (or an alternate solution) to merge the new user locale setting and locale switching to WP.com.

[swissspidy]

See also #38590, #29892

[pento]

In 39235:

I18N: Add the get_available_languages filter.

Sometimes, a language file may not exist in exactly the format or location that get_available_languages() expects it to be in - for sites with this level of customisation, they need to be able to add their own language files to the list of those available.

Props yoavf.
Fixes #38788.

[ocean90]

In 39244:

Docs: Apply documentation standards to the new get_available_languages filter.

See #38788.

[ocean90]

Replying to pento:

Sometimes, a language file may not exist in exactly the format or location that get_available_languages() expects it to be in

For this case, wouldn't it make more sense to have a pre_get_available_languages filter instead?

[pento]

Replying to ocean90:

For this case, wouldn't it make more sense to have a pre_get_available_languages filter instead?

Either works, I think. @yoavf is doing the migration, so I went with his recommendation.

[yoavf]

@ocean90 no strong preference. I think that the current filter serves this purpose, but potentially another purpose of keeping the original list but excluding specific languages.

[neoxx]

#38591 was marked as a duplicate.