Support language translations in Payment link page + invoice PDFs

🔍 Problem Statement

As an Italian- or German-speaking user, I experience invoice PDFs, payment link labels, and related error messages in the wrong language, which results in confusion during billing and payment workflows.

💡 User Story

As an Italian- or German-speaking user, I want invoice PDF labels, payment link labels, and billing-related error messages to respect the selected site language, so that I can understand payment details and complete billing actions confidently.

🎯 Definition of Done

A feature is done when:

✔️ Given the site language is set to Italian, when an invoice PDF or payment link is generated, then all invoice labels, payment link labels, and relevant error messages are displayed in Italian.

✔️ Given the site language is set to German, when an invoice PDF or payment link is generated, then all invoice labels, payment link labels, and relevant error messages are displayed in German.

✔️ This change will impact UI / PDF generation / Billing / Payment Links, specifically:

1. Invoice PDF label translations

2. Receipt PDF

3. Payment link label translations

4. Billing and payment validation/error messages

5. Localization files or translation resources

6. Any shared billing components used by invoices or payment links