Steps to reproduce:

1) On the clean eZ Platform install language pack, for instance fr_fr (or any other locale, which uses , decimal separator instead of .
2) Configure your browser primary language as French [fr].
3) Create a new class with a float filedtype
4) Create an object of this class and insert float (12.34 for instance) value and publish it
5) In the preview, you should see the correct value in the field.
6) Edit content. Field input will be empty. In browser's developers tool you can see that value is like <input .. value="12,34" instead of <input ... value="12.34".