language selection not respected throughout

[René J.V. Bertin]

Hi,

As you may know I've been building KF5 libraries and applications for installation into /opt/local, on both Linux and OS X. 
I had already noticed that my en_GB language setting wasn't respected systematically and throughout, which at first I put off on missing/incompete/imperfect translation files. It got somewhat better when I installed plasma-workspace and could use the Regional KCM to apply my language preference "globally". At least I no longer had to repeat the setting for each new application. 

Then I checked from my wife's account on the Linux box; this account is configured to talk French. Most individual applications do indeed appear in their French translation, but the KCMs continue to use en_US just like they do for me. Same for KWin5 which I set as the default WM on my wife's account: it uses en_US in the menus. I double-checked: the language files are all there in /opt/local/share/locale.

For the record: both systems have an appropriately configured KDE4 install from which the language settings could/should have been migrated!

Am I missing something? XDG_DATA_DIRS contains /opt/local/share, of course.

Thanks,
R.