I have done this successfully a few times, but it means that all language settings (Project Tree > Languages & resources > Project languages) in the whole project must be English (United Kingdom). The default is English (United States).
If you change the language on a heavily commented project, you need to move everything you've typed using the Project texts resource. This brings up a table and everything from the newly added language will be blank.
After you've done everything, untick English US and make sure both Editing language and Reference language are changed.
Very important to take a backup before you do this, it affects all HMI labels, tag names and I think rung comments too.