How To Change Default LANG on Kali Desktop Environment

As a root, you can edit /root/.dmrc and make a change on the parameter Language under [Desktop] to, for example, en_US.utf8

Finally it should look like:

Language=en_US.utf8

Prior to this, you may also need to generate the locale en_US.UTF-8 using below command as a root:

dpkg-reconfigure locales