The level of locale support is undocumented. I would have included this in my documentation feedback but when I did a test I found that the implementation is also incomplete - i.e. it only appears to impact the decimal point specifier. Here is a simple reproducer (both output lines should be the same):

                    std::locale a("en_US.utf8");

                    std::cout.imbue(a);

                    std::cout << 1122.89 << std::endl;

                    constexpr boost::decimal::decimal_fast64_t val4{ 112289, -2 };

                    std::cout << val4 << std::endl;