If you want to change the language of your installed SimCity game, you can do so by uninstalling and reinstalling your game. Below you can find step-by-step instructions on how to do this:
Log into the Origin client.
Uninstall SimCity by right clicking on the game in your My Games section, and selecting "Uninstall"
A new Window will appear. Select your Game from the list and click "Uninstall".
Follow the on-screen instructions.
After successfully uninstalling SimCity, change your Origin client language by going to Origin>Application Settings> General. Here you can select a new language under "Origin Client Language"
Choosing your new language will cause your Origin client to restart and you will need to log back in.
In the My Games section of the client mouse over SimCity and click on "Download". A new window will appear where you can choose your desired language. After the Download has finished the game will be installed automatically.

The next time you start the game it should run in the correct language. Please be assured that with reinstalling your game no progress will be lost.

Please note: If you purchased the Russian, Polish or Hungarian version of the game, you will not be able to install the game in English. If you would like to play the game in English, you will need to purchase the English game version from Origin or other online retailers.