The ASCII standard only contains 128 characters, therefore what you are calling an 'ASCII' file is not. You are probably referring to ISO 8859-1 (commonly known as 'Latin-1') which contains all the latin characters required to write the german language.
The thing is, unicode is a superset of both ASCII and ISO 8859-1, so in theory a valid ASCII file is also a valid unicode (UTF-8) file.
UTF-16 and UTF-32 are different in the way they represent their characters, so would not be valid ASCII documents.
Without seeing the files in question it is hard to make any further comments.