Load fallback locale when needed if lazy-loading (fixes #34)

[rchl]

Fallback locale is needed to properly handle strings that exist in
base/fallback locale but not in currently used one.

Fix from @orblazer

[codecov]

Codecov Report

Merging #291 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #291   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           2      2           
  Lines           4      4           
=====================================
  Hits            4      4

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[paulgv]

Thanks for picking this up @rchl! Thanks for submitting the initial fix @orblazer!