FormatCommandLine wrote option and value arguments using the ambient
thread culture via value.ToString(), while the parser reads them back
with ParserSettings.ParsingCulture (invariant by default). On threads
whose culture uses a comma decimal separator (for example de-DE) a
double 1.5 was unparsed as "1,5" and reparsed as 15, and a sequence
[1.5, 2.5] became "1,5,2,5" and reparsed as four elements [1,5,2,5],
so the format/parse round-trip was broken.
Format IFormattable values (numeric and date/time types) with the
parser ParsingCulture instead of the current thread culture, for both
scalar and separator-joined sequence values. Other types keep their
default string representation, so behaviour on invariant/en-US is
unchanged.
Add round-trip tests under de-DE for a scalar double and a
comma-separated double sequence.
dualfroz