Disable locales that have a completion rate below 40%

[diox]

Frontend part of mozilla/addons#15433

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.28%. Comparing base (51186d1) to head (f254d7b).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #13482   +/-   ##
=======================================
  Coverage   98.28%   98.28%           
=======================================
  Files         267      267           
  Lines       10610    10610           
  Branches     3233     3233           
=======================================
  Hits        10428    10428           
  Misses        169      169           
  Partials       13       13           

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