[olo@olo-pc ~]$ pacman -Qs l10n
local/calligra-l10n 3.0.0.1-2 (localization)
Localization for Calligra
local/kde-l10n-en_gb 16.12.2-1 (kde-uninstall kde-l10n localization)
KDE language support: British English
local/plasma-l10n 5.9.1-1 (localization plasma5)
Language packs for Plasma 5
[olo@olo-pc ~]$ pacman -Qs i18n
local/firefox-i18n-pl 51.0.1-1 (localization)
Polish language pack for Firefox
local/ki18n 5.31.0-1 (kf5)
Ki18n
local/perl 5.24.1-1 (base)
A highly capable, feature-rich programming language
local/thunderbird-i18n-pl 45.7.1-1
Polish language pack for Thunderbird
I've just run Octopi, installed kde-l10n-pl and now all works fine except login screen, date there is still in English.