Source

File: wp-admin/includes/class-language-pack-upgrader.php

public function clear_destination( $remote_destination ) {
		global $wp_filesystem;

		$language_update    = $this->skin->language_update;
		$language_directory = WP_LANG_DIR . '/'; // Local path for use with glob().

		if ( 'core' === $language_update->type ) {
			$files = array(
				$remote_destination . $language_update->language . '.po',
				$remote_destination . $language_update->language . '.mo',
				$remote_destination . 'admin-' . $language_update->language . '.po',
				$remote_destination . 'admin-' . $language_update->language . '.mo',
				$remote_destination . 'admin-network-' . $language_update->language . '.po',
				$remote_destination . 'admin-network-' . $language_update->language . '.mo',
				$remote_destination . 'continents-cities-' . $language_update->language . '.po',
				$remote_destination . 'continents-cities-' . $language_update->language . '.mo',
			);

			$json_translation_files = glob( $language_directory . $language_update->language . '-*.json' );
			if ( $json_translation_files ) {
				foreach ( $json_translation_files as $json_translation_file ) {
					$files[] = str_replace( $language_directory, $remote_destination, $json_translation_file );
				}
			}
		} else {
			$files = array(
				$remote_destination . $language_update->slug . '-' . $language_update->language . '.po',
				$remote_destination . $language_update->slug . '-' . $language_update->language . '.mo',
			);

			$language_directory     = $language_directory . $language_update->type . 's/';
			$json_translation_files = glob( $language_directory . $language_update->slug . '-' . $language_update->language . '-*.json' );
			if ( $json_translation_files ) {
				foreach ( $json_translation_files as $json_translation_file ) {
					$files[] = str_replace( $language_directory, $remote_destination, $json_translation_file );
				}
			}
		}

		$files = array_filter( $files, array( $wp_filesystem, 'exists' ) );

		// No files to delete.
		if ( ! $files ) {
			return true;
		}

		// Check all files are writable before attempting to clear the destination.
		$unwritable_files = array();

		// Check writability.
		foreach ( $files as $file ) {
			if ( ! $wp_filesystem->is_writable( $file ) ) {
				// Attempt to alter permissions to allow writes and try again.
				$wp_filesystem->chmod( $file, FS_CHMOD_FILE );
				if ( ! $wp_filesystem->is_writable( $file ) ) {
					$unwritable_files[] = $file;
				}
			}
		}

		if ( ! empty( $unwritable_files ) ) {
			return new WP_Error( 'files_not_writable', $this->strings['files_not_writable'], implode( ', ', $unwritable_files ) );
		}

		foreach ( $files as $file ) {
			if ( ! $wp_filesystem->delete( $file ) ) {
				return new WP_Error( 'remove_old_failed', $this->strings['remove_old_failed'] );
			}
		}

		return true;
	}