When searching for data in ArcCatalog by its geographic location, the extent information recorded in the metadata is used if metadata exists. Because of a known problem in the Microsoft XML parser that is used by ArcCatalog, searches that perform numeric comparisons do not work with numbers that use commas as the decimal symbol.
Therefore, ArcCatalog must store extent information in the metadata using numbers with periods for the decimal. As a result, real numbers that are added to the metadata automatically by ArcCatalog always use periods for the decimal place even if another character is defined in your regional settings as the default decimal symbol. In addition to the data's extent, this issue affects the numbers describing the spatial reference's parameters, and the size of the dataset.
This will not affect your ability to add numbers with commas for the decimal symbol using the metadata editor. However, keep in mind that because of this issue custom stylesheets also can't do numeric comparisons on numbers with decimal symbols that are not periods.