It's not exactly a decimal point, more a "decimal mark" as per ISO 8601. From the Wikipedia article for the standard at  "https://en.wikipedia.org/wiki/ISO_8601#Times -

"However, a fraction may only be added to the lowest order time element in the representation. A decimal mark, either a comma or a dot (without any preference as stated in resolution 10 of the 22nd General Conference CGPM in 2003, but with a preference for a comma according to ISO 8601:2004) is used as a separator between the time element and its fraction."

and the citation is

"ISO 8601:2004(E), ISO, 2004-12-01, 4.2.2.4 ... the decimal fraction shall be divided from the integer part by the decimal sign specified in ISO 31-0, i.e. the comma [,] or full stop [.]. Of these, the comma is the preferred sign."

Nothing about picking a decimal point based on the current locale.