Yesterday I found out the hard way that (beyond measurement units) I never use localized PMReal, so finding WidgetUtils took a while and I still had no good answer to your sub-question.
Again, is there any widget that exposes a similar behaviour? Then you could create an instance of that boss and use its ITextValue. That overkill is probably not the best solution, though.
Another direction, short from platform specific code, could be boost.
And if you read that page, you'll also eventually run across a reference to "ICU". Yup, the SDK has a very promising folder "external/icu". I think we're coming closer - happy digging.
I am missing some thing. Would be a case of just change the ',' to a full point and convert in the usual way.
Conversion between internal values and strings (either for display or entered by the user) should follow the local setting for decimal separator (and, if necessary, digit grouping). It's up to the OS to provide this information.
As a matter of fact, here in Holland some of our computers are set to the Dutch system -- comma as decimal separator, period as digit group separator -- but it depends on the software if that is recognized for input or output. I don't think Adobe's software as a whole conforms to this, so it's always a guessing game.