Download the plugin Extract all the files. Upload everything (keeping the directory structure) to the /wp-content/plugins/ directory. Activate the plugin through the ‘Plugins’ menu in WordPress.
For a quick start, you can look at Polylang, getting started PDF document. It will help you to setup your multilingual website step by step with a lot of screenshots. To go further, the full documentation is available below:
When you edit or add gallery into content.