unicode defines a start/middle/end part: https://github.com/unicode-org/icu/blob/fd123bf023882f07bfacf51c39111be2f946d8f8/icu4c/source/data/locales/en.txt#L2008-L2019

this was on my list to investigate :) not sure it fits String component, as i was already skeptical about $glue->join($strings) for an API.

ultimately i want to format list values in ICU translation files, but the spec format is missing still :( https://github.com/tc39/proposal-intl-list-format

@ro0NL I'm not trying to solve 100% of the problem but the most common that we've all been solving again and again. TC39 is another level of complexity (same reasoning about title(true): it's not linguistically universal, yet it's common enough to deserve being supported out of the box.)

What do you think about making the method static, like in Java: https://docs.oracle.com/javase/8/docs/api/java/lang/String.html#join-java.lang.CharSequence-java.lang.CharSequence...-

For me, it is a kind of named special constructor for a string (and not a method for doing something with the glue string).
$str = UnicodeString::join($array, $glue, $lastGlue);

I think it would make the method harder to discover.

I'm not even sure this should belong to a String component, as it is more about dealing with arrays...

@stof Python has it. That's a serious enough precedent to me.