toUpper is definitely language-dependent. For example, in Irish there are initial letters that are written as lower-case even in all caps. Wikipedia's example is amusing, since it's a photo of a government passport office sign - the all-caps version of Oifig na bPasanna is OIFIG NA bPASANNA (photo
https://upload.wikimedia.org/wikipedia/commons/thumb/f/f8/AL..., article
https://en.wikipedia.org/wiki/Irish_orthography#Capitalisati...). It would look utterly bizarre to write OIFIG NA BPASANNA. And this isn't at all an unusual construction in Irish, it happens in personal names all the time.
Plus, there's the issue of diacritical marks. Irish keeps long marks over capitals, but French drops accents. Do you plan to do é => É (required for Irish - POBLACHT NA hÉIREANN is the all-caps version of Poblacht na hÉireann [the Republic of Ireland]) or é => E (common practice for French)? You have to get it right, and you have to know the language to do that. (Poblacht na hÉireann also illustrates the fact that initial caps is also a language-dependent idea; you absolutely can't write Poblacht Na Héireann - that makes my eyes burn just looking at it.)
(And before you say, well, Irish isn't a language spoken by very many people, remember that it's an official language of the European Union. If you're writing software to be used by EU agencies, you're going to have to care.)