here's our FAQ on this.
Q: After the update to Nokia Belle, my phone language has changed and I can’t find my native language in the settings anymore. What should I do?
A: A software update resets the language settings of your phone to the state in which the phone left production originally. That means, as an example: If your phone was originally produced for the Finnish market and had some additional languages like Chinese added manually later on, after the update to Nokia Belle your phone will not show the extra Chinese language anymore. If you have such a case, please consult your local Nokia Care point (locations via www.nokia.com/support).