Have your default locale set to 'Slovenian' which uses a comma for the decimal symbol, like 2.76 is written as 2,76. Observe that the format specifier 'n' does not work for the default 'Slovenian' locale setting.

Then try running this in the Python interpreter:
'{number:.3n}'.format(number=2.76)

You get this:
>>> '{number:.3n}'.format(number=2.76)
'2.76'

Excepted the is the return value '2,76'. Please fix this bug.