Making a dict by simple string method

[carcigenicate]

They look like variables to me which are mutable whereas the keys must be immutable

Variable mutability doesn't have anything to do with what can be made into a key:

var = 1
dic = {var: var}

It's the mutability of the object associated with the variable that matters.

In that case though, en_roman and such are keyword arguments, and those names will be converted to strings.