Currently Blender only accepts the period symbol as decimal separator.
But this is only the standard in about half the world. A majority of European countries, like Spain, France, Norway, Denmark, and more use comma for this. As does parts of Africa, Indonesia, Mongolia, Venezuela, Brazil, Chile, Argentina, Colombia, and Uruguay, and some Quebecers.
This patch doesn't strictly make you choose, so don't think this is more disruptive than it is. This patch generally allows you to use both symbols for decimal separator. It is mostly about how the result is shown. So you can set "comma" for decimal separator yet still use the "." on your keypad for this.
Once applied you can go into Preferences and change to "comma":
Once you make this change you can enter numbers using comma as decimal separator or with period, but what is shown will be with commas. And with this set any large integers that are normally shown as "1,234,567" will be shown as "1.234.567" as is the custom in almost all places that use comma decimals. I have kept this patch simple by not allowing for differences in integer grouping. So India does not get 2,3 groups and Switzerland and Liechtenstein do not see apostrophes. Easy enough to add these options but I liked the simplicity of having just one switch.
No, it does not add much complexity at all. Most of this patch is dealing with large integer grouping, not with decimals.
Yes, all the fancy things we do in number inputs still work. I have found only one thing that necessarily has to work differently. If you have decimal "point" selected then you can enter "1,5" to get "6", while obviously that will give you "1.5" if set to decimal comma.