You’re supposed to know all about this issue already. You probably do know it. You most likely think it’s not a mistake you’d ever make. It’s the kind of thing that ought to belong in Elementary Dunglish but, surprisingly or not, it remains the commonest simple mistake of all.
English uses a decimal point. (The comma is a thousands delimiter.)
Both EN-US and EN-GB.
End of story.
- Writing “1,2” instead of “1.2” is plain wrong.
Admittedly, the cases where it could be misinterpreted and create confusion are few and far between, though not inconceivable. Does “1,500 mg” mean a gram and a half or a milligram and a half measured to microgram accuracy?
But it is an error that grates, instantly telling the native speaker reading the text that this is a European author whose English may be ropey.
Prevalence: very high. Endemic, even: over half the documents corrected needed this correction at least once.
Frequency: very high. when present in a document, the correction was needed on average seven times. And that’s just in running text, excluding the many bulk corrections in tables and diagrams and formulae.
Native: never. Not a mistake English speakers make either in text or when translating.