Split locale into <language>_<country> and go looking for keyboard
layouts that match. Do that in reverse, so look for country first.

- known weakness is el_CY [should get layout gr] because CY and el
  don't name any keyboard layout.
- known weakness are Hausa, Igbo .. which are ha_NG and ig_NG. They select
  keyboard layout ng, which is labeled 'English [Nigeria]'; they ought
  to select ng[hausa] and ng[igbo], which are the right variant keyboard
  layouts to use.
- similar selecting a locale in Canada [en_CA, fr_CA, iu_CA ...] will
  select keyboard layout ca, which is for French-speaking Canada.
  Locale en_CA should select keyboard en -- e.g. en[us]. But iu_CA
  [Inuktituk] needs layout ca[ike].