If possible, numbers should be stored in a format that works across cultures. This can be done by storing them in binary so culture is irrelevant, or by storing them as text strings using a single known culture (preferably the Invariant culture). This way you always know how to read in the numbers from your file(s) and can use any other culture you want to display those numbers in your application.
Documentation for Decimal.ToString(IFormatProvider)[
^] - Used to write a decimal value using a specific culture. Other numeric types and String.Format() have similar overloads using IFormatProvider.
Documentation for BinaryReader class[
^] - Used to read in data from binary files.
Documentation for BinaryWriter class[
^] - Used to write data to binary files.