I have US English as the default language of the system and also of my user account, but I've set a Polish keyboard layout exclusively (I'm writing in both and standard English alphabet is just a subset of the Polish alphabet, but English is missing like 20% of Polish letters: ąćęłńóźż). When I'm using Numerical Keyboard and press decimal separator (little 'Del' key) I get a coma (,) instead of a dot (.). The dot is a decimal separator in English, coma is used for this in Polish. That happens in native GNOME 3 apps (e.g. Gedit, Empathy), Chromium, Thunderbird, Firefox, Mendeley but NOT in LibreOffice (if I remember correct, LibreOffice has it's own setup of this feature).
I suspect it is keyboard setup problem, as it is the only non-English thing set in my system. In KDE you can change decimal separator arbitrary.
How one can arbitrary change decimal separator in Unity flavored Ubuntu?
Just in case my locale:
A bit on the side. When using Kubuntu derivative distro I had some issues with locale too. Some apps used my user settings to account for the language they displayed (KDE native apps), some other (Chromium, Thunderbird) used the system settings. if you set them the same, you don't notice that, but when they are different you have a bad time.