"And your argument about comma separators is wrong; the string

1,234

in a CSV file SHOULD mean "two values: 1 and 234", regardless of the local decimal separator."

Yes, I agree, it SHOULD mean that, but that is NOT what Excel does when the decimal separator is set to "," in the regional settings. Excel wrongly thinks it should apply the comma as decimal separator, and reads that number as 1 unit and 234 thousands.

Locale MUST NOT be used for data formats, but Excel does it anyway.

This problem doesn't manifest itself when you're using a locale which matches the CSV's separators. Consider yourself lucky if you're in that situation.