Source

File: wp-includes/l10n.php

function get_available_languages( $dir = null ) {
	$languages = array();

	$lang_files = glob( ( is_null( $dir ) ? WP_LANG_DIR : $dir ) . '/*.mo' );
	if ( $lang_files ) {
		foreach ( $lang_files as $lang_file ) {
			$lang_file = basename( $lang_file, '.mo' );
			if ( 0 !== strpos( $lang_file, 'continents-cities' ) && 0 !== strpos( $lang_file, 'ms-' ) &&
				0 !== strpos( $lang_file, 'admin-' ) ) {
				$languages[] = $lang_file;
			}
		}
	}

	/**
	 * Filters the list of available language codes.
	 *
	 * @since 4.7.0
	 *
	 * @param string[] $languages An array of available language codes.
	 * @param string   $dir       The directory where the language files were found.
	 */
	return apply_filters( 'get_available_languages', $languages, $dir );
}