Unfortunately, there is no button to change the language on Turbotax. 

You would have to log out and on the top right corner, you would be able to see the language switch button or simply follow the following link  TurboImpôt. 

Thank you for using TurboTax

Unfortunately, there is no button to click to change it to French in the downloaded version of the software, but you can always download TurboImpôt, which will allow you to use the FR instead.

Please note that whatever tax return you are processing on your computer, will be in both versions of the software when you open either TurboTax or TurboImpôt. To a maximum of 20 returns per computer.

When the register button pops up simply click on the X to close that window.

For the online users, you can log in to TurboTax top right corner click on "Sign In" or TurboImpôt top corner click on "Se connecter".

Thank you for contacting TurboTax.