What would be a good alternative for strings internal to your program?

I work with multilingual text processing applications, and I strongly support that concept. A guideline of "use UTF8 or die" works well and avoids lots of headaches - it is the most efficient encoding for in-memory use (unless you work mostly with Asian charsets where UTF16 has a size advantage) and it is compatible with all legal data, so it's quite effective to have a policy that 100% of your functions/API/datastructures/databases pass only UTF8 data, and when other encodings are needed (e.g. file import/export) then at the very edge of your application the data is converted to that something else.

Having a mix of encodings is a time bomb that sooner or later blows up as nasty bugs.

Abstraction is the alternative. Design an API that treats encodings uniformly, and the encoding becomes an internal implementation detail. You can then have a polymorphic representation that avoids unnecessary conversions. NSString and Swift String both work this way.

Vector of pointers to grapheme clusters for example.

Sometimes vector of objects that include other information, like glyphs etc.