Changing locale by setting about:config (prefs.js) does not work

[pscdodd]

Setting extensions.fxkeyboard.locale_default to the string "de", "sl" etc. does not change the keyboard from the "en" layout to "de" or "sl".
Firefox version: 56.0.1 (64 bit)
Windows 8.1
Extension version: 2.4.2.1