Fix duplicate locale when creating an entry without specifying the locale

[petersg83]

Replace #10256

Description
With the Content-Manager API It was possible to create a related entry in the default locale, even if the default locale was already existing. To do it, you just had to create a new locale without specifying the locale.

[codecov]

Codecov Report

Merging #10378 (bb18c3e) into master (55eaa91) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #10378   +/-   ##
=======================================
  Coverage   57.84%   57.84%           
=======================================
  Files         184      184           
  Lines        6395     6395           
  Branches     1394     1394           
=======================================
  Hits         3699     3699           
  Misses       2233     2233           
  Partials      463      463           

Flag	Coverage Δ
front	∅ <ø> (∅)
unit	57.84% <ø> (ø)

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