UNICODE_STRING is still experimental and there is currently no support built-in support for UTF8 strings.

They're bot beyond the feasibility, so I thing we should rely on external libraries.

There are a *lot* of libraries to handle Unicode as it is a fairly complex standard.

Regarding UTF8 "apt search utf8 dev" finds https://github.com/sheredom/utf8.h which seems quite nice.
There is also GLib, but using it would make each program compiled by Liberty dependent of it.

BTW, €, ü and à are correctly recognized as not being CHARACTER, as it is mapped to a C char, which is an 8 bit value. And according to stores 7 bit.