One of the examples are in the srcset HTML attribute used on images, generated at Html::srcSet. The value is cast to float and then to string, causing locales with comma as decimal separator (like es_ES or fr_FR output 1,5 instead of 1.5. Such conversions should be done with PHP number_format.
I wonder if it would be safe to simply set setlocale(LC_NUMERIC, "C") somewhere on the code. Who knows where else it may be happening.
Reported originally in Support desk - Comma Instead of Period in SRCSET Breaks IMG Tag