fix: improve Windows development setup and language selector

[RSAgr]

Overview

This PR addresses two issues encountered when setting up the development environment on Windows.

Changes

1. Docker Build Fix - Added default USER_ID=1000 and GROUP_ID=1000 to the Dockerfile, as the Makefile's id command doesn't exist on Windows
2. Language Selector Fix - Changed language codes from "en-us"/"pt-br" to "en"/"pt-br" to match Django's standard locale codes, fixing the language dropdown that was reverting to default "pt-br". Also, by default now english is the chosen language (though the backend still doesn't translate the page to any other language chosen)

Testing

• Docker build successful on Windows
• Language selector now properly switches between English and Portuguese

Related Issues

Fixes issues with Windows development setup

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