feat: Sign up request with the country and the language of the user

[g123k]

Hi everyone!

Currently, all users are registered with the "world" website, and they receive the newsletter in English.
Before implementing a way to change the language dynamically, we now ensure to send the user's language and country to fit 95% of cases.

[codecov-commenter]

Codecov Report

Merging #4329 (4b5adc8) into develop (5a72a46) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           develop    #4329      +/-   ##
===========================================
- Coverage    10.54%   10.54%   -0.01%     
===========================================
  Files          290      290              
  Lines        14293    14295       +2     
===========================================
  Hits          1507     1507              
- Misses       12786    12788       +2     

Impacted Files	Coverage Δ
...th_app/lib/pages/user_management/sign_up_page.dart	49.78% <0.00%> (-0.44%)	⬇️

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[github-actions]

You can test this PR on: https://app.waldo.com/applications/app-19d476740ba1bb36/sessions

[g123k]

Thanks, @monsieurtanuki for the review.
Let's merge this ASAP, so we don't receive complaints anymore ;)