A 'character' meaning what? The first code point? The first non-combining code point? The first non-combining code point along with all associated combining code points? The first non-combining code point along with all associated combining code points modified to look like it would look in conjunction with surrounding non-combining code points along with their associated combining code points? The first displayable component of a code point? What are the following?

the second character of sœur (o or the ligature?)

the second character of حبيبي (the canonical form ب or the contextual form ﺒ ?)

the third character of есть (Cyrillic t with or without soft sign, which is always a separate code point and always displayed to the right but changes the sound?)

the first character of 실례합니다 (Korean phoneme or syllabic grapheme?)

the first character of ﷺ or ﷽ ?

The main issue isn't programming language support, it's ambiguity in the concept of "character" and conventions about how languages treat them. Imagine how the letter i would be treated if Unicode were invented by a Turk. The fundamental issue here is that human communication is deeply nuanced in a way that does not lend itself well to systematic encoding or fast/naive algorithms on that encoding. Even in the plain ASCII range it's impossible to know how to render a word like "ANSCHLUSS" in lower case (or how many 'characters' such a word would have) without knowledge of the language, country of origin, and time period in which the word was written.