In case you have a multi-language store and want to add Store Codes to the URLs to make them look like e.g. yourdomain.com/en, yourdomain.com/fr, yourdomain.com/au follow next steps.
Navigate to Stores > Configuration > General > Web and find the Url Options section. There you choose to Add Store Code to URLs selecting Yes from the corresponding select box and Save Config.
Note: Store Codes are set while creating store view manually in Stores > All Stores. So in order for store codes to be added to URLs, you need to make sure you've set them for each store view.
Admin Panel Configuration is just one of the series of articles about How to Configure Multi-Language Magento Website? which include information about the creation of a new store view, language packs installation, useful multi-language store extensions, product&category, CMS pages&blocks, email templates, and other translations in Magento 2 website. You should definitely get acquainted with them in order to know how to manage a multilanguage store.