Created attachment 112756 [details] screenshot My system language was pt-br, so I added american english and got the error message shown in my screenshot. It happened on both Arch Linux (plasma 5.13 beta) and neon dev unstable.
in plasma 5.13.80 (up to date neon-devedition-gitunstable) i have no "i18n_argument_missing" when I add a language
This is a true code bug, looking into it.
Fixed with https://cgit.kde.org/plasma-desktop.git/commit/?id=150f8877ee42f0d7f69c90549b4d8cca578de4ea