In my opinion, and as a general rule. You can't really ever truncate a user facing string correctly because it depends on language specifics. Hence it is my suggestion not to truncate user facing strings - and in fact you may want to consider them as binary blobs. On the other hand, as a matter of practicability sometimes you may have to, but certainly avoid doing so before it hits the presentation layer, and know that it may not be language correct.
There are many strings that are not user facing, which you expect to be of a certain nature, e.g. ASCII based protocols, and therefore you know what to do with them.
So the multi byte situation and strcpy, or std::string, or any other "standard" string function isn't really relevant as it's some other libraries duty.
the task of truncation and otherwise formatting UI strings is the preserve of the rendering layer(s).
Yes, truncating a user facing string requires more consideration regardless of programming language. For example do you truncate at the grapheme, word, sentence, new line, paragraph, or something else? How do you indicate truncation to the user, with an ellipsis perhaps? If you use an ellipsis is it appended to the original string or drawn by the GUI toolkit?
Note that the Unicode grapheme cluster, word, sentence, and line break algorithms are locale specific. Now consider how often programmers casually truncate strings, even in high-level languages, without accounting for the locale.
There are many strings that are not user facing, which you expect to be of a certain nature, e.g. ASCII based protocols, and therefore you know what to do with them.
So the multi byte situation and strcpy, or std::string, or any other "standard" string function isn't really relevant as it's some other libraries duty.
the task of truncation and otherwise formatting UI strings is the preserve of the rendering layer(s).