Performant Translations: PHP translation files erroneously in language dropdown

[swissspidy]

This is a follow-up to #59656 and specifically [57516], which caused get_available_languages() to loop through not only .mo files but also .l10n.php files.

This caused odd "de_DE.l10n.php" entries in the language dropdown that obviously won't work.

Trac ticket: https://core.trac.wordpress.org/ticket/60553

[swissspidy]

In 57639:

I18N: Prevent incorrect language dropdown entries when there are .l10n.php files.

In [57516], the just-in-time translation loading logic was enhanced to support cases where only .l10n.php translation exist but no .mo or .po files. This caused a slight regression in get_available_languages(), which uses the list of files to populate the language dropdown list on the settings page.

To address this, the new file extension is now properly stripped off, and the resulting file list is de-duplicated. New test files are added to allow the existing tests to cover this new scenario.

See #59656.
Fixes #60553.

Committed in https://core.trac.wordpress.org/changeset/57639