Fix splitting of en locale codes according to DeepL requierements

[unreadableusername]

Added American and British English locales to translation target locale array.
fixes #738

[unreadableusername]

I see the checks fail, but judging from the output this has nothing to do with my small change, but rather with some dev dependencies that might need a bump right? I am just anxious because this is my first contribution 🥳

[zerolab]

We have some Netlify jobs that fail and are on the list to be replaced. Triggered the actual CI tasks (they require approval to be run on PRs from first time contributors)

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (e1051b0) 92.65% compared to head (5a78a67) 92.65%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #739   +/-   ##
=======================================
  Coverage   92.65%   92.65%           
=======================================
  Files          46       46           
  Lines        4002     4002           
  Branches      596      596           
=======================================
  Hits         3708     3708           
  Misses        174      174           
  Partials      120      120           

Files	Coverage Δ
wagtail_localize/machine_translators/deepl.py	66.66% <0.00%> (ø)

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[zerolab]

Added tests and merged in a3efa8f
Thank you