Retrieves languages available during the site/user sign-up process.

function signup_get_available_languages() {
	/**
	 * Filters the list of available languages for front-end site sign-ups.
	 *
	 * Passing an empty array to this hook will disable output of the setting on the
	 * sign-up form, and the default language will be used when creating the site.
	 *
	 * Languages not already installed will be stripped.
	 *
	 * @since 4.4.0
	 *
	 * @param string[] $languages Array of available language codes. Language codes are formed by
	 *                            stripping the .mo extension from the language file names.
	 */
	$languages = (array) apply_filters( 'signup_get_available_languages', get_available_languages() );

	/*
	 * Strip any non-installed languages and return.
	 *
	 * Re-call get_available_languages() here in case a language pack was installed
	 * in a callback hooked to the 'signup_get_available_languages' filter before this point.
	 */
	return array_intersect_assoc( $languages, get_available_languages() );
}