I can't change the values because I can't confirm them.
Took me a while to work out what "can't confirm them" meant, but now I understand: if the entered field text is not a number, then the OK button is disabled.
The problem is still likely that the "," is not accepted by IsNumber as a decimal point. So, two things to try are:
1/ temporarily set your localisation to something that uses "." as a decimal point, see if that changes the keypad "," to a "."
2/ with localisation set back to your region and using "," as a decimal point, see what IsNumber("123,45") and IsNumber("123.45") return
It might be possible to hook into the EditText that is part of the dialog and convert "," to ".", but... it'd be far better if the number verification accepted the local decimal point character.
In the meantime, perhaps you can use a text input but with the numeric keypad. Yeah, your users will then be able to confirm/Ok invalid numbers, like with two decimal points/commas, etc, but you can catch that by fixing/checking the returned value with .Replace(",", ".") / IsNumber().
What could possibly go wrong?!?!?!
Looking forward to the results of checks 1/ and 2/.