On the Sites sidebar panel, there’s a button for adding a new site to your content, “Add site”. It lets you provide a new site with a homepage in a selectable language. A site created in this way is a stand-alone site in the sense that it is not a language version of any other existing site.
For adding a site as a language version, an “Add language” menu item or button is available. With stand-alone sites (that aren’t a language version), “Add language” can be found in the context menu of the corresponding element on the sidebar panel, whereas a group of sites representing language versions features an “Add language” button directly on the panel.
Like “Add site”, “Add language” also allows you to select a language, but the dialog additionally lets you specify a source site from which the content of the new site should be partly copied. This is convenient if the source content is a good starting point for creating the content of the new site, e.g. by translating it. Note, however, that only top-level pages will be copied over from the source site. These are the pages that usually show up in the main navigation.