I believe the source code of the JDK should be in US-ASCII if possible, and only employ extended characters if that is strictly necessary for the code to work.

In my attempt to figure out which non-ascii files are another encoding than utf-8 (see JDK-8301971), I discovered a handful of files that use unicode characters for no good reason, when normal ASCII characters could have been used (and have been used everywhere else in the code base in similar contexts).