This might be dependent on your country or locale setting... as opposed to the keyboard you use. Are you in a country which uses a comma as a decimal? Might be worth quizzing someone who is - test my app or setting your locale to french or similar and seeing what happens (as long as you can get back to your own locale in that language!!)
Or create your own soft keyboard.
What are you trying to achieve? i.e. what is the context of your question.