A couple of thoughts on this…
First, I use the period-as-decimal/ commas-as-thousands convention, so I’ve not had to deal with this. I’m sure you know you can fix this as a text element using “:formatted as…” , so that could solve your display for monthly rate.
For the input value Annual Rate, I’m not sure there is a clean fix. I couldn’t find a simple solution on the forum. There was a discussion here Question: US vs. European number formatting (please HELP) of allowing a selection of conventions that would standardize it app-wide, but I’m not sure anything ever came of it.
One workaround would be to have the annual rate as a dropdown in 0.5 or 0.25 increments displayed the way you want using a datatype with a number field and a text field. It displays the text field to the user (i.e. 1,5%) but uses the number field for the calculation (i.e. 0.015)