Why AI Models Get Locale-Specific Number Formatting Consistently Wrong

AI language models frequently produce malformed numbers when generating locale-specific output, such as writing '1,234,56' in German by applying the decimal comma rule while ignoring the corresponding shift to a period for thousands grouping. The decimal and thousands separators are a matched pair, and applying only one half renders the number unparseable. The problem is compounded across locales: French, Russian, Polish, and Swedish use non-breaking spaces as group separators, which break standard string comparisons and regex patterns written against ordinary spaces. Swiss German adds a third distinct pattern — apostrophe grouping with a decimal point — while Indian English uses non-uniform digit grouping under the lakh-crore system. The International Bureau of Weights and Measures recommends thin spaces for digit grouping in scientific documents precisely to avoid ambiguity between decimal and grouping characters across conventions.