UTF-8 is explicitly designed in such way that unix API dont have to care about it and such that lexicographic ordering of utf-8 encoded byte streams is same as lexicographic ordering of unicode code point vectors (which arguably almost never is what you care about when sorting text strings). Both of these features are result of conscious design and not some random coincidence.

As for sorting text strings of any kind (which has mostly nothing to do with unicode), you have to care about what user expects, which is dependent on locale and sometimes even on user's preference. Algorithms to do text sorting are for each separate locale mostly non-trivial, but fortunately can be generalized into one relatively straight-forward algorithm (similar to unicode normalization) involving pretty large database and some special cases.

When you are sorting bit strings you are sorting lexicographically, but that is usually not what you should be doing.