In the solution explorer, expand the HMI, Extensions and ADS tabs respectively and double click on ADS. This will bring up the 'TwinCAT HMI Server Configuration' window. You can also open this window by selecting 'TwinCAT HMI' on the menu bar, then 'Windows' and finally 'TwinCAT HMI Server Configuration'. 

Select the 'TcHmiSrv' on the configuration window and make the necessary changes as shown below: 

Scroll down on this window and set the language for user groups as well. 

Set the default language in the HMI properties too.