We are using Power BI Report server to publish Power BI Reports internally. And being in Australia we use a “sensible” date format of dd/mm/yyyy 
However at a recent showcase with some of our end users they commented that our date format was “backwards” ie. mm/dd/yyyy
If you are using US regional settings on your PC you probably have not come across this issue, but for other regions this can be a problem.
Upon getting back to my desk I checked the report in Power BI Desktop and the date format was correctly picking up my local regional settings. I then opened the report in IE (which is what most of our End Users have as a default browser) and the report was still using the Australian format (dd/mm/yyyy):
However when opening the same report in Chrome or Firefox the date was using US regional settings (mm/dd/yyyy):
After doing some google/binging I found that both Chrome and Firefox have their own internal regional settings dialog and they both default to using US settings.
If you go into the Settings menu in Chrome and search for “Language” you will find the following, and it’s not just the default display language that matters the order is also important. Moving “English (Australia)” to the top of the list, then deleting cached images and files fixed this issue.
Clicking on the 3 dots to the right of the language gives you an option to move that language to the top.
Firefox has a very similar language dialog and the same fix works for it too.
What this does behind the scenes is to change the “Accept-Language” headers that the browser sends to PBIRS to make your preferred language first in the list. This in turn alters the regional formatting setting javascript file that is sent down to the browser. Before this change I could see a file called angular-locale_en-us.js being used, after the change I now see angular-locale_en-au.js