Here are some answers:

Whether the currency symbol is to the left or right of the price

AFAIK, the currency symbol is to the right side of the price: 123 €

Which symbol to use as thousands separator

Use a thin space for number with more than 3 digit long (i.e. 1 000 €, 10 000 €)

Which symbol to use as decimal point

ALWAYS use a comma: 123,45 €

Whether the rules also cover the formatting of foreign currencies

This is valid for any currency and also for any unit: 12 345,67 kg

Reference Académie Française (especially §3)

To complete @M42 answer,
I’ll add that we should use 1,23 € but use a lot 1€23 in France, to write it like we say it. In particular, the latter is used on many price tags.