docs(i18n): make translation guide more explicit

[villebro]

SUMMARY

Improve the translation section in the new docs:

• Move the instructions for creating a new language file before the other steps, including instructions to skip to the next section if the target language already exists
• Add link to ISO 639-1 language codes
• Add instructions for using poedit under MacOS
• Make the finalization steps more clear by adding a section "Applying translations"

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

1. Go to docs-v2
2. Run 'yarn install'
3. Run yarn start
4. Navigate to http://localhost:3000/docs/Contributing/translations in your browser to check the updated docs

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[geido]

LGTM!