At the moment it is not possible to customize currency fields as a developer would expect. Under specific circumstances we have the following behaviour:
When you have a custom entity with a lookup on currencies and a currency field, the symbol that is shown in the currency field will only orient on the lookup if the "default currency" in the personal settings of the user is empty. When the user has chosen a "default currency" in the personal settings, this currency symbol will be shown. Even with Javascript I cannot change it.
So it can happen that the lookup currency is EURO but the symbol in the currency field is $ (when the user has chosen USD as default currency).

I already create a MS TIcket and this was the resolution:
"Our developers confirmed that the symbol (like $, €, etc.) will change on fields of type currency only if you update it in the personalization settings. But the "Currency" field in the Quote record has nothing to do with that symbol and will not affect or be affected by the value in the personalization settings."

This makes absolutely no sense to me. That is why I propose to make the currency symbol somehow chooseable or curstomizable.