feat: flag European-style switching of commas for decimals and periods for thousands separators

## Description

English uses the period `.` for decimals: `3.141592654` and the comma `,` for thousands separators: `$1,000,000,000`
But many European languages do the exact opposite. Many Europeans don't know this or forget it when writing in English and write instead `3,141592654` and `$1.000.000.000`

## Resources

- [Wikipedia: Decimal separator](https://en.wikipedia.org/wiki/Decimal_separator)

## Examples

- **3,141** - What is that? Pi or Three thousand one hundred forty-one? I'm sure it's Pi!
- For example, 1 million can be written: English: 1,000,000.00; Spanish: **1.000.000,00**; ISO 31-0: 1 000 000.00
- Worse still: in Germany we officially separate thousands with the decimal point, one million would be **1.000.000** (or **1.000.000,0** as a float)

## Potential False Positives

One potential gotcha is with Indian English, where `,` is used a bit differently to English because in large numbers some groups can be two instead of three. This is related to the use of lakhs and crores in Indian English numbers.

The issue names no source files, tests, or entry points. Start by locating existing numeric-format checking rules and their tests, then determine how decimal and thousands separators are represented and distinguished from Indian English and other valid usage. Done means the intended European-style switching is flagged without the listed false positives.

Written by the indexing model from the issue text.