Does this have to do with the Language and/or date/time format?
If so, I can confirm that HEAT will automatically select the language based on related settings on your computer. If HEAT is displaying/defaulting to English US, it is caused by a mis-configuration on the computer (which is what IE on Windows or Safari on Mac uses) or web browser settings Chrome/Firefox etc. Chrome/Firefox seem to install with English US as the default language in my experience.
To directly answer your question: I have found no way to set the default locale for all users....perhaps because of what I said above (it's designed to choose it automatically).
I was shown a case on an Android Mobile device where the option to change the Locale in Self Service was there, but it didn't hold its setting.
Luckily we don't officially support such devices, but it is a sad situation.
This didn't make a difference for one of our clients. Still got US English.