One of the strength of TYPO3 is the ability to support multi-language website. There are a couple of things to do for adding a new language. Very roughly for this website:

Configuration for the Frontend

• Add a record sys_language in the Backend by clicking:
  Menu web > List > New TYPO3 site > look for section "Website Language" > click the "green" button
  
  
• Adjust the TypoScript configuration to your needs. Configuration is to be changed in both files:
  /htdocs/typo3conf/ext/speciality/Configuration/TypoScript/Config/language.ts
  /htdocs/typo3conf/ext/speciality/Configuration/TypoScript/constants.txt
  
  
• There is likely to configure the language menu (for displaying the new language). Configuration is in file:
  /htdocs/typo3conf/ext/speciality/Resources/Private/Partials/LanguageMenu.html
  
  
• Real URL - the extension generating the speaking URL - must be updated as well. Open htdocs/typo3conf/RealURL.php and look for "Language configuration" within the file.

Configuration for the Backend

• In the Backend, the default language is configured as English. This can be changed in
  /htdocs/typo3conf/ext/speciality/Configuration/TsConfig/Page/config.ts

There are also some resources for editor that can be useful.

• Open the Language module and download the languages of your choice by clicking:
  Admin Tools > Language
  
  
• Apply language for the Backend by clicking:
  User Tools > User settings > Menu "Language"

Default Backend language can also be pre-configured for a user by clicking:

• Admin Tools > Backend users > edit a user