Yeah I think the legacy interop is the tricky part, see "Why does C# use UTF-16 for strings?" (http://blog.coverity.com/2014/04/09/why-utf-16/#.V1AT9vkguUl) for example.

Interestingly enough Java just implemented compact strings that can be ISO-8859-1/Latin-1 (one byte per character) or as UTF-16 (two bytes per character). See http://openjdk.java.net/jeps/254 and https://www.infoq.com/news/2016/02/compact-strings-Java-JDK9 for more info.