Language specifications includes date format AND translations.
If you want to change a specific text in the translation, there is a tricky method here: https://askubuntu.com/a/253396/650621 (for ubuntu unity, but the solution should be similar)
But if your only problem with the US locale is date format, you should install Raspbian in en_US and then there is an option to change locale for time and date only and maybe also numbers, not sure. It should be in
Preferences->Language Support->Regional Formats. (Or environnement variable LC_TIME if you prefer to change it in command line)
Raspbian uses LXDE as desktop manager, you will have more luck if you search on the Internet how to do something LXDE, it's how I found the answers above.