2 following terms should be distinguished:
- default site language (language which was used for creating the site)
- alternate site language (language which is used for displaying site for end users)
Default site language is the language which was used for creating the site. Once site is created its default language can't be changed. However it is possible to add so called alternate languages to the site (go to Site settings > Language settings > set Enable pages and news to be translated into multiple languages to On and then add needed languages to the list). On Sharepoint on-prem you would need to install language pack for that but on Sharepoint Online it is available automatically.
So e.g. if site was originally created with English and then you added German as alternate language - in this case if user will visit your site and browser's language setting of this user is set to German then your site will be displayed on German language to this user. More information about multilingual UI in Sharpeoint can be found e.g. here: Plan for the multilingual user interface in SharePoint Server.