Eli Zaretskii wrote:
>    emacs -Q
>    M-x set-variable RET unibyte-display-via-language-environment RET t RET
>    M-: (set-buffer-multibyte nil) RET
>    C-q 0242 SPC
> 
> This should display ¢.
> 
> So I think we can get rid of making echo-area buffers unibyte, as long
> as we make sure that variable is nil (which it is by default).

Getting rid of it sounds good, but why do we need to worry about 
unibyte-display-via-language-environment? For me, the attached patch does 
display that test as ¢, and it fixes the other test cases reported so far in 
Bug#30405. And yet this patch works without worrying about 
unibyte-display-via-language-environment, even if I run Emacs in a unibyte 
locale like en_US.iso885915 (a practice that's no longer common).

For what it's worth, I'm testing on Ubuntu 16.04 and on Fedora 27, built 
--without-imagicmagick so that I can reproduce the original problem.