Format Strings

C/C++: there's a good summary in cppreference
C#: these are introduced here and the references for numbers, dates and such are described here along with custom formats. These are all typically culture-aware, although you can use a culture-invariant formatter. Standard formats are things like long dates with specific arranges, while custom formats are things like year/month digits or strings in custom orders.
Python: the mini-language is described here.