Feat(unlock-app): Support localization on checkout with i18n

[njokuScript]

Description

This PR implements internationalization with the i18n package on the checkout.

Tasks

• [x] Setup i18n on app globally
• [x] Refactor checkout codebase to use the translate method
• [x] Support English and Spanish

Issues

Fixes #9519 Refs #

Checklist:

• [x] 1 PR, 1 purpose: my Pull Request applies to a single purpose
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have updated the docs to reflect my changes if applicable
• [ ] I have added tests (and stories for frontend components) that prove my fix is effective or that my feature works
• [x] I have performed a self-review of my own code
• [x] If my code involves visual changes, I am adding applicable screenshots to this thread

Release Note Draft Snippet

[julien51]

@SVell I wonder if we should split this PR and start with the the Setup i18n on app globally?

[SVell]

@SVell I wonder if we should split this PR and start with the the Setup i18n on app globally?

For the whole unlock-app?

[julien51]

For the whole unlock-app?

Yes. The goal is the make changes more incremental so we can very easily add more languages later.

[julien51]

more specifically, do a PR with the changes in i18n (but empty lang folder) + package.json, then a second one with the changes in the checkout and finally a PR for each new language added to lang.

[julien51]

Let's close this one as it is currently stale and would probably require a lot of changes.