I feel like you might be underestimating the difficulty of localization. There are way more customary ways to write numbers then you can think of (there is probably a Falsehoods Programmers Believe About Numbers out there which addresses the innumerable ways people write numbers).

I also feel like you are underestimating the amount of localization that the browser actually does. Yes you still have to do some localization, but there is a whole issue that the browser does for you if you use the correct attributes. There are way more upsides which you might be unaware of.