Ok, so here is the deal. Android cannot deal with the comma as a decimal separator due to problems in the OS. These bugs may be fixed in later releases of Android, but that won't help most phones that are out there now. If you simply use the "period" as your decimal separator, will the app work on your phone set to Swedish ? Or does it fail in that manner too.
No, it will fail.
The built in calculator app uses a decimal point, overriding the locale setting, so there should be a workaround for this problem.
Found this for another android app:
OnTrack has been updated to 1.8.2 in the Android market. This version is a minor update with the following changes:
In some locales the comma is used as the decimal separator instead of the period. Unfortunately there is a bug in Android 1.5 that doesn't allow users to enter a comma in these locales for decimal fields. The problem that arose is when editing an entry, OnTrack would copy the value into the edit field with a comma making it an invalid number. OnTrack now automatically changes the comma to a period to workaround this Android issue. In all all other places in OnTrack a comma should be displayed if this is what your locale uses as the decimal separator.