Add some i18n e2e tests

[petersg83]

Merge #10255 before.

• Add many e2e tests
• Make a small fix. 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, it was need to not specify the locale in the query.

[codecov]

Codecov Report

Merging #10256 (1ab07ef) into master (311d8e1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #10256   +/-   ##
=======================================
  Coverage   57.86%   57.86%           
=======================================
  Files         183      183           
  Lines        6348     6348           
  Branches     1392     1392           
=======================================
  Hits         3673     3673           
  Misses       2215     2215           
  Partials      460      460           

Flag	Coverage Δ
front	∅ <ø> (∅)
unit	57.86% <ø> (ø)

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[petersg83]

There is an issue with the tests that is difficult to fix in V3. As the code won't change before the V4. We won't add those tests, knowing that they passed.
The detected issue is fixed there : #10378