As a very first step, please make a Transfluent user account if you don't have yet. Sign-up here: SIGN IN
Please contact our support, and we will send you a zip-file and API-key. The zip-file you can extract to /wp-content/plugins. Once installed, go to Wordpress plugins and enable it.
After enabling the plugin, go to the settings and insert your Transfluent users' email and API-key as well as map the languages between Polylang and Transfluent. Remember to click "save settings"!