A term for "radix point" that everyone would understand?

Solution 1:

Based also on your further comments I interpret your question as primarily "how to specify the decimal separator so all users clearly understand", my answer is rather practical than linguistic :)

Is there a better term for "decimal separator"?

I don't think so.

Is there a better way to phrase the statement to make it clear what we're asking?

Sure, here you have so many possibilities! It depends on the meaning of the number. You may also include a few examples to make it really clear.

E.g.:

Enter your height in meters, use "." as decimal separator, e.g. 1m 72cm as "1.72"
Enter the price you request, use "." as decimal separator, e.g. 15€ 50cents as "15.50"

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