> for instance, Python now uses a weird mixture of ASCII and UCS2 internally.

Really? Last I heard (PEP 393), the rule was: "8 bits if all codepoints are less than 256 (i.e. Latin-1); 16 bits if all codepoints are less than 2^16 (i.e. BMP); otherwise 32 bits". This means that text with all Latin-1 characters (which are approximately the first 256 codepoints of Unicode) will be stored internally as, well, Latin-1. This implies that ASCII strings are stored as ASCII.