I think that convention depends on the language, not the currency.

For example, in German it's usually written postfix, but in Dutch it's usually prefix.

That actually varies by country. In Ireland it is used as a prefix

Only in some countries. The position of the currency symbol is a locale thing

It very much is a locale thing, not even a country thing - e.g. en-ca and fr-ca have '$' as a prefix/suffix, respectively.

Not universally

> Placement of the sign varies. Countries have generally continued the style used for their former currencies. In those countries where previous convention was to place the currency sign before the figure, the euro sign is placed in the same position (e.g., €3.50).[7] In those countries where the amount preceded the national currency sign, the euro sign is again placed in that relative position (e.g., 3,50 €).

> In English, the euro sign – like the dollar sign ⟨$⟩ and the pound sign ⟨£⟩ – is usually placed before the figure, unspaced,[8] the reverse of usage in many other European languages

> The European Union's Interinstitutional Style Guide (for EU staff) states that the euro sign should be placed in front of the amount without any space in English, but after the amount in most other languages

https://en.wikipedia.org/wiki/Euro_sign#Use

In Austria it's prefix.

I just trust whatever the LC_CURRENCY settings do.