Using Plasma with:

$ cat $HOME/.config/plasma-localerc
[Formats]
LANG=pt_BR.UTF-8

[Translations]
LANGUAGE=pt_BR:en_US

$ setxkbmap -query
rules:      evdev
model:      pc101
layout:     us,br
variant:    intl,
options:    terminate:ctrl_alt_bksp

The above configuration should produce "ç" and "Ç" by typing, respectively, '+c and '+C as per /usr/share/X11/locale/pt_BR.UTF-8/Compoose.

Instead, the above key combinations produces "ć" and "Ć" as per /usr/share/X11/locale/en_US.UTF-8/Compose.

Note that even if /usr/share/X11/locale/en_US.UTF-8/Compose is changed to produced the ccedillas as above, the setting is ignored and the default behavior still take place. It looks that /usr/share/X11/locale is totally ignored.

A user defined workaround may be set with:

$ cat ~/.XCompose:
include "%S/en_US.UTF-8/Compose"
<dead_acute> <C> : "Ç"
<dead_acute> <c> : "ç"

But that shouldn't be necessary if Plasma do its locale definitions properly.

As a side note: There is "Brazilian Portuguese", "European Portuguese" and such. They could be spelled as "Portuguese (Brazilian)", "Portuguese (European)" for sorting purposes in other languages, or translated to Portuguese as "Português do Brasil" and "Português Europeu". KDE staff have declared that Brazilian Portuguese (pt_BR.UTF-8) is a standard one, which is not.