If you are handling multilingual text the locale is mandatory metadata.
The lowercase of "DON'T FUSS ABOUT FUSSBALL" is "don't fuss about fußball". Unless you're in Switzerland.
It is probably time for an Esperanto advocate to show up and set us all straight.
What are you talking about? In Esperanto, one letter equals one sound.
Se fareblus oni, jam farintus oni. (It definitely won't happen on an echo-change day like today, either. ;))
Contra my comrade's comment, Esperanto orthography is firmly European, and so retains European-style casing distinctions; every sound thus still has two letters -- or at least two codepoints.
(There aren't any eszettesque bigraphs, but that's not saying much.)
If you're printing a name, you're probably printing the name for the current user, not for the person who entered it at some point. If you're going to try to convert back like that, you also need to store a timestamp with every string in case a language changes its rules (such as permitting ẞ instead of SS when capitalising ß). And even then, someone might intend to use the new spelling rules, or they might not, who knows!
This article probably boils down to "programmers don't realise graphemes aren't characters and characters aren't bytes even though they usually are in US English". The core problem, "text processing looks easy as long as you only look at your own language", is one that doesn't just affect computers.
Your best bet is to just avoid the entire problem by not processing input further than basic input sanitisation, such as removing whitespace prefixes/suffixes and maybe stripping out invalid unicode so it can't be used as a weird stored attack.
islower is actually supposed to account for the user's "locale", which includes their language.
The key takeway is that lowercasing a string needs to be done on the whole string, not individual characters, even if std::string had a way to iterate over codepoints instead of bytes (or code units, in the case of wstring).
And there isn't a standard way to do that, you either meed to use a platform specific API, like the windows function mentioned, or use a library like ICU.