By default, the web interface language type matches the web browser locale, but you can override that in your user preferences. Click the menu next to your user name and select Preferences then choose your language option. If you change your language setting, that won’t take effect until the next time you sign in.
The user interface language for the desktop and mobile apps is set automatically based on the user locale set for the operating system. You can’t override this language setting in the apps.
See Supported Software, Devices, Languages, and File Formats in Administering Content and Experience for a complete list.