chore: translate strings on the sign up screen

Description

Translate strings on the signup screen

[ ] ✨ New feature (non-breaking change which adds functionality)
[ ] 🛠️ Bug fix (non-breaking change which fixes an issue)
[ ] ❌ Breaking change (fix or feature that would cause existing functionality to change)
[ ] 🧹 Code refactor
[ ] ✅ Build configuration change
[ ] 📝 Documentation
[x] 🗑️ Chore

[x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
[x] I read and ran all relevant commands as specififed in the Running Tests section of the Contributor Guide.
[x] The title of the PR follows the Conventional Commits guideline
[x] My local branch follows the naming standards in the Deepsource Branch Naming Convention or Biodiversity Branch Naming Convention
[x] I listed at least one issue that this PR fixes in the description above.
[x] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy,
[x] I updated/added relevant documentation (doc comments with ///).
[x] I added new tests to check the change I am making, or this PR is [test-exempt].
[x] All existing and new tests are passing.