Do error messages really have to justify themselves?

[0xEAB]

Do error messages really have to justify themselves?

Error: implicit string concatenation is error-prone and disallowed in D

And why does this message specify the programming language?
Or: Why don’t other error messages do so?

[Geod24]

Because it was originally a deprecation message and was never changed. Also because people might expect it to work based on C syntax.