At the moment Jolla supports the following languages for the user interface:
Eesti keel (Estonian)
Español (Bolivia) (Spanish (Bolivia))
Português Brasileiro (Brazilian Portuguese)
Tiếng Việt (Vietnamese)
繁体中文（台湾） (Traditional Chinese (Taiwan))
繁体中文（香港）(Traditional Chinese (Hong Kong))
Most of these languages have a dedicated keyboard.
To change your device language:
1. Select "Settings"
2. Then scroll down until you see the Globe icon and press it:
3. Select your language from the list of all languages - they are all presented in their native language:
4. You will need to confirm the language change by tapping the top-right corner OR by swiping left:
5. Your device will restart and the selected language will be in use.
To read more about configuring keyboard languages, please see here for more.