You don't handle localization in the field directly. You let it be "text" and ask your preferred library (which is built by people smarter than you/me) to conve

You don't handle localization in the field directly. You let it be "text" and ask your preferred library (which is built by people smarter than you/me) to convert said text to valid number. If it converts then good, now you have a number variable in memory with which you can do whatever. If it fails you pop-up the error message and ask the user to enter a valid number. I find on the fly correction (don't let user type letters, ignore signs, etc) to be confusing to end-user.