The metric system still fails to impose itself universally, so we can wait some time before the decimal mark is unified...
English: Use commas for separating thousands, use periods for decimals.
Metric: Use spaces for separating thousands, use periods OR commas for decimals.
In this instance, the English approach is obviously the superior one, as spaces are more confusing to use as a delimiter than commas, and there should only be one way to write decimals for consistency.
However, I think that must be a convention from before the universal adoption of the metric system by the EU, as the 22nd General Conference on Weights and Measures (held in 2003) declared that the symbol for decimals should be "the point on the line or the comma on the line".
i.e. the official metric system symbol is period or comma, regardless of what actual Europeans use.
http://www.bipm.org/utils/en/pdf/Resol22CGPM-EN.pdf [page 11]
Regardless? Those (and mainly comma) is exactly what Europeans use. It's not like there is some other symbol besides those too in use for that purpose.
What you are talking about is a European convention, not a metric convention. There is a difference.
Yes, and the grand-parent already said that this particular case is insignificant compared to English vs Metric.
Let's first fix this, and then we worry about decimal separator.
Another important difference is that in Europe we understand a billion as a million of millions (10^12), not a thousand of millions (10^9). For us, a 10^9 is a miliard. Luckily I´m also aware of the use of billions for 10^12 and whenever I read it on a US or UK text I understand correctly. Otherwise, saying that there are 7 billion people on Earth would be an extreme exageration...