Google has updated its review snippet structured data help documentation to recommend using dot separators in numerical review values over comma separators in numerical review values. Both dots and commas will work, but Google is recommending the dots over the commas.
Google added this paragraph twice once for the reviewRating.ratingValue property and once for the ratingValue property:
For decimal numbers, use a dot instead of a comma to specify the value (for example 4.4 instead of 4,4). In Microdata and RDFa, you can use content attributes to override the visible content. That way, you can show the user whatever style convention you want, while also satisfying the dot requirement for structured data. For example:
Google said that if you are currently using comma separators for decimal ratings in your markup, you'll still be eligible for review snippets. However, Google now recommends that you update your markup for a more accurate interpretation.
Forum discussion at Twitter.
